It should be called, unless! the application's main thread contains a loop in such a way that it never idles.
Note that I have not used fpGui for a quite a long time - I really should use it more - but afaicr it worked at least under linux 64 (Debian flavors)
We need to have some sourcecode that replicates this
We also need to know which platform, OS, CPU, bitness, versions. "It doesn't work" is not very informative.