ULF film holders, keeping them flat...

Question for ULF users, any troubles with your ULF film holders ?
Got three of them (24x17") and they do not seem to like to stay completely flat.
The camera spring back press them enough, but I do not feel this is a good answer !
I wonder if this is commun with ULF holders....
Will much appreciate different advise.
Thanks,
MAC

Hello mac ,

(24x17")

What a strange size ?

oupsssss !!!!
mean 14x17"
:)