Three things it's likely to be: impeller, clogged/encrusted water journals (not too likely if you flush it regularly), clogged telltale supply tube.
If you haven't had the impellers changed in the past couple of years, it's time to have it done (on both motors.) If you have, it may be wise to have the weak one done anyway (it's cheap insurance.)
You can remove the cowling, ID where the telltale comes out of the cowl, and trace the feed back to the block. Disconnect the tube there (it's going to spray water all over), and compare that stream to the other motor. If they're the same, you can blow compressed air or a high pressure stream of water from the telltale back through the tube to clear it. If the streams differ, it's the impeller or the water journals are being obstructed somehow.
Water journals: You could probably try running it in a tub with a bunch of Salt-Away or similar, but I would suggest taking it to a shop and having it flushed professionally. Obstructions are not necessarily salt.
Hey, wait a minute... An Angler isn't a RIB...
jky
