Yes check your panel specs. Most of the GE panels that support THQP breakers only support them in the bottom slots, the upper slots are not intended to be used with 1/2 height breakers (the connectors on the bar actually look different on the slots where they are ok to go). They also seem to be a bit "wigglier" than the full height breakers, and "felt" better when the 220 breaker was sandwiched between two 110's so both slots were full (or from the bottom - 110, 220, 220, 110) so that there wasn't any gap (not having an open gap to the rail is smart anyway).
Jason: The double pole THQP breakers are meant to straddle two slots, not fit in one slot (they can fit in one slot.. but don't work as noted). The breaker itself is bonded so if one leg trips both will - which isn't really what you want for two 110 circuits. The THQP 110v breakers are 1/2 the height of a full slot whereas the 220 are equivalent to one full height breaker.