It's a known design problem.
Your best bet it the knob pull trick. The knob itself is probably not broken....but because of the way it's designed, the shaft is probably out of alignment with the button that switches effect types.
If the knob pull doesn't work, I'd suggest mailing the M5 to Line 6 or a service center. It is within the Line 6 warranty period and should be fixed for free.
When ya gonna wake up Line 6??? The design is borked.
- Admit it.
- Offer free repairs for all existing owners.
- Come up with a new design for the next iteration of M-Series ASAP.
- Offer a nice discount for existing owners that trade up to the new model.
- Make a darn big deal of it....your customers and the community will applaud you and you'll end up making even MORE money.
That's what REAL companies like Apple and Amazon do.