Most likely cause: Air leak on the suction side — even a pinhole at a flange gasket or packing will prevent prime.

Diagnostic steps:
1. Fill the casing manually and close the discharge valve. Does it hold prime for 30 seconds? If not, you have a suction air path.
2. Inspect suction flange gaskets and packing — tighten or replace any that show weeping.
3. Verify suction pipe rises continuously from source to pump — any high point traps air and kills prime.
4. Check NPSHa vs. NPSHr at current flow — if NPSHa is marginal, you'll lose prime under load even with a perfect seal.