I have never had any problem with a similar (Delta) mortiser. I suspect your bits are not sharp. Often, they are simply not honed fine enough out of the box.
For the outer bit, the chisel, the outsides need to be sharpened flat and polished to about 600g+ and the inner portion sharpened with a cone type sharpener in a hand drill, usually the finest grit you can fine--Lee Valley has a good one. Of course, the inner drill bit needs to be sharp as well. The mortiser also has to be set up so the drill bit hogs out the first 1/8th of an inch or so, not the chisel. Finally, some wax or glide coat on the outside of the chisel will help things move along nicely.
Here is a good video on how to do all of this:
https://www.finewoodworking.com/2014...mortising-bits