As long as you have the intakes submerged or can verify it’s definitely getting a good supply…and you’re then not getting water up it…that assumed, one might think that it may be clogged with salt deposits further up or it’s not together right and drawing air somewhere.
If it’s salt deposits…I just recently managed to clear out an old Yamaha with the same issue. Believe it or not, I took the lower unit off and turned the thing upside down almost…then poured vinegar into it to soak…after a wee while…I then got a small plastic syringe that came with kids medicine or something…and injected some more vinegar into the tell tale outlet. Did that a few times and eventually it cleared it all out.