Check by hand to see if any air is escaping, I have the same problem, if necessary tighten the screws and check the seals in the head. For the moment, it is working reasonably well again.
Test 1: Reinsert the bottle, make sure the thread runs smoothly and screw tightly. Attempt 2: the fizzer may be blocked or clogged (sweet drinks fizzy). Rinse for a long time and keep rinsing and rinsing. If none of this helps -> service