Weak massage is a plumbing question before it is a jet question. We measure what the manifold delivers, then work outward to the seats.
Jets die three ways in this valley: passages narrowed by mineral, air controls gummed until no fizz reaches the seat, and diverter valves that crack and quietly send the flow elsewhere. All three feel identical from inside the tub, which is why we start at the manifold with a pressure check instead of unscrewing jet faces at random.

The good news is that most jet work is mechanical and quick once diagnosed — a flushed venturi line, a freed control, a diverter gate, a handful of inserts with fresh tabs. The expensive version is only expensive when a cracked body has been leaking into the foam, which turns a jet job into a leak job.
System-wide starvation — filter, pump output or scaled passages. The manifold reading says which.
Diverter valve, almost always. The jets behind it are usually innocent.
Venturi lines or the air control, plugged with mineral. Flushable in most cases.
Worn locking tabs on the insert. New insert, thirty seconds, no cabinet work.
Real faults, real fixes — the paperwork version, minus the addresses.
Diverter gate had split. Valve, not jets — the jets were fine all along.
Venturi lines gummed shut with mineral. Flushed, controls freed, fizz back.
Describe the symptom and the brand — the diagnosis starts on the phone and finishes with one written number.