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Not familar with Prolinica...
It's an old Elinchrom line, made in England at one time. I suspect the OP is talking about something like these monolights:
I have just acquired two old Prolinca studio heads which both operate on a half or full power switch. One head works perfectly but the other only fires on half power regardless of the switch setting.
When you say "head" do you actually mean a strobe head that connects to a power pack (AKA "generator"), or do you mean a self-contained monolight? (See photo below.)
In any event, Chuck's answer is almost surely the correct one: failure of one of the capacitors or its triggering circuit.