Diagnosis
This is a classic symptom of a clogged or scaled-up boiler outlet or the one-way valve. The initial steam pressure pushes a little water through, then the clog stops flow.
How to Fix It
Descale aggressively. Use a stronger solution (e.g., 2 parts water to 1 part white vinegar) and let it sit in the cold machine for 30 minutes before turning it on to brew. Run the full cycle, then rinse with two cycles of clean water. This clears mineral blockages in 95% of cases.
If That Didn't Work
The heating element or its thermostat may be failing, not bringing the water to a full boil to create proper pressure. This requires professional diagnosis with a multimeter.
Heads Up
Using very hard water without regular descaling will cause this. If you have hard water, descale every 1-2 months as preventative maintenance, not just when it fails.