It's pretty common for the 1.6 VW diesels to not start when hot.
It seems to always point back to a weak starter.
Seems the engine gets tighter when hot, harder to turn.
A new starter has always fixed it for me.
Another work around is to have the glowplugs come on even when hot.
With a 1.6D I just need to disconnect the glow plug relay temp sender.