Modern Vespa

Hi. New to the group and new owner of a vespa gts, bought it as a none runner, I've now got it running and very pleased with it after owning over many years both vespa and lambretta 2 strokes.
The problem I have now is after a short run and engine warm it won't start after switching off, no problem whatsoever when cold but once warm starter engages but it's as if engine is locking up and a loud clack is heard, I suspect the decompression valve ?? Almost as if engine is hydraulicing , too much compression and nowhere to go ? Any one had experience with this fault and rectification. Many thanks

Finally starts after it cools down or after a couple tries? If you just got it running i'm assuming new battery?

Hi thanks for responding, no it's not electrics, all that side of things are in order, my suspicion is decompressor, has anyone had issues with this, or fitted a Malossi head and cam upgrade?