I have to agree with Jeff, based on experience and use.
Wear, dubs edges. Stropping might not remove enough material to easily correct this dubbing, but if done properly and regularly, stropping can even slow the dubbing caused by wear. I strop the moment I think the blade is not perfect. Two to four strokes on a side does not take long and keeps tools pretty nice. Paradoxically. the longer you wait between stropping, the more you will dub the blade.
An ideal edge is keen. Such as this side view of an edge on a sharpening surface.
As the edge gets worn down and corrected, and as we rock the blade while stropping or honing, the edge starts to recede from the surface.
In the illustration above, without rocking the blade, or grinding down and eliminating the secondary bevel, sharpening on a hard surface means the edge is not going to contact the sharpening media. If you rock the edge, you are as likely to shift the angle even more and this will accelerate dubbing.
If you are sharpening and sharpening and not getting any results, then this is the probable reason, and you need to regrind the edge to true before you can make the edge keen.
A somewhat flexible media, used with a gentle touch will allow for this, without requiring an increased angle. If you strop with an increased angle, you will accelerate dubbing, if you push down hard, you will accelerate dubbing.
Oddly enough a touch of dubbing is not such a bad thing. It strengthens the edge. A dubbed edge can still be keen. The problem is that as it wears down, eventually you will need to either bear down (heaven forbid) or start to shift to a higher angle in order to contact the actual edge and return it to keen.
The effect of stropping and wear in combination could be considered similar to a micro-bevel. A fairly obtuse edge can still cut fibers, as long as it is keen. If you look at the illustration above and think of it as a dubbed blade in use instead of a stropped edge, you can see where wear on a well stropped, slightly dubbed blade, can to a certain degree correct the previous wear damage.
Wear caused by use, combined with regular low pressure stropping at low or no angle, can allow you to continue using an edge for quite a long time. Proper stropping means contact with abrasive, but no strong distortion of the blade or edge. The actual abrasives involved are tiny, so the pounds per inch of force applied as the abrasive drags across the blade can be quite large despite the gentle touch being used.
If when you strop, you stroke the edge more side to side than you drag it base to point, you will tend to produce a smoother, more unified edge and therefore keener edge.
If when you strop, you find yourself needing a higher angle to get results, then it is time to hone or even grind your blade back to true.
Ideally you want a hard, flat slab of leather for your strop.
If you take a section of vegetable tanned leather, preferably horse butt, wet it and then let it dry to the point where it still feels cool and damp, but looks dry, then you can compress it between two flat plates in a vise and turn it into a very hard smooth leather surface. Hammering, at this moisture level, was the traditional method to compress leather for making shoe soles, but this can give an irregular surface that will need to be sanded down before used on a strop. After this drys completely, this can then be glued to a flat board with the flesh side up. The smooth skin side is ok, but the rough textured side is preferred for stropping.
Plain, ungritted leather will strop to as fine an edge as you might want, but will not be as fast as a strop that has been treated.
With fine monocrystalline diamond grit, working it into the leather while it is still wet will make for a faster strop that will hold up a long time. Rubbing a
good honing compound on the surface also works wonderfully. I keep my strop barely supple with some non-drying oil. Camellia, olive or Ballistol all work well for this.
When using a strop after you hone, to remove feathers, you should start out flat on the strop with pressure just hard enough to maintain even pressure. The term 'kissing the leather' is pretty much dead on. Raise the angle a bit as you discover the feathers are not coming off. The higher the angle, the more dub, but the faster the feathers are removed. So you want the lowest angle that will flex the feathers enough and wear the feathers base enough for them to come off.
After the feathers are gone and you are touching up the edge, start out stropping with the bevel flat against the strop, take about four strokes on each side and test the edge. If it is not sharp enough, then raise the back of the blade a bit, take four strokes on each side and try again. Once you find the right angle stropping should be pretty fast. Almost all of my blades will sharpen fine, flat on the strop. A few seem a bit more resistant to stropping and need a higher angle. Some only need a couple of strokes to get back to a keen edge.
All of this said, it is very hard to beat a fresh bit of unadorned and untreated vegetable tanned tooling leather for stropping. Just lay it on the table flesh side up and a few gentle even strokes with the flat of the bevel parallel to the leather. This is how I take care of my leather working tools, and they stay wicked sharp. I think the original skin, and perhaps the materials used in tanning put enough of a polishing agent in to allow for a pretty good strop, at least until those materials wear down.
Bob