Could be a number of things, cheapest of which is a duff tank cap, ie if not sealing well it lowers the boiling pint enough to cause this, a pump not circulating, how is the heater ?, how long does it take to boil the water, thermostat.
You can have perfect compression even with a blown HG, as it depends where it has blown.
I would start with a new stat, after checking the cap and that water is circulating.
tabetha