I have built quite a few layouts using Flexbed and have had good success with diameters down to 045. The 045 took a bit more coaxing than the larger curves but it did eventually lay flat. Of coarse I have been using Atlas or Gargraves where the ties are close together and help hold the bed in place once the track is screwed down. I have never glued the bed in place. It seems I am always modifying track plans or adding switches and would personally hate dealing with bed that had been glued down.
I am guessing that you are using tubular track and with the large spaces between ties, I could see why it would buckle.
If I were trying to use it with 031 curves and having the issues you are, I would first try cutting each section down the middle (much like cork roadbed is sold) and then installing each side under the track. This should allow much tighter curves without the inside wanting to buckle.
Good luck and happy railroading,
Don
