If you have a Jabsco manual head they are notorious for losing prime. Take off the inlet on toilet, and with garden hose force water through until it's filled. Replace the hose to toilet being careful not to spill too much water and see if that works. If not a rebuild kit might be in your future.
It's most likely the flapper valve at the top of the pump, item #21 on page 8. They have a different name for it, but for all us old-timers, it's a flapper valve.
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