The decompresser can be a temperamental part, however swapping it for new should make i all good.
Did you use a new or a used decompression valve, and did you notice any differences between the old and new ones?
If it's a new valve, I would focus more on spark, or the vacuum line to the valve, however, defineatly check the valve and do the no-valve test, as I have seen new ones with faulty diaphragms.
