Digital BW, The Print

I do my own framing, and printing to A3+ 13 x 19. I have had some 
prints done at the 24 x 36 size and I cannot get them to stay flat 
once framed. I have tried minimal fixing via tape the matt board and 
maximum by using spray fix to the foam core backing. Either way they 
move and wrinkle and are thus unusable. Bear in mind that where I live 
it's really hot, sometimes hot and dry and some times hot and humid. I 
live in the middle of now where so whatever solutions are offered try 
and keep that in mind.....

Thanks