When a pump starts to prime, but won’t fully
prime (i.e., you can see air bubbles under the lid), it’s
likely that the system has a small leak; and it’s usually on
the suction side.
Switch the whole system off, and look for a small drip or squirt of
water from the pipes, outlets, sweep hoses, and O-rings on both the
suction and return lines.
You may have to switch the system on and off several times to catch
the dripping leak, but it’s almost always the cause of this
— Al Mangin, owner, Dependable Pool Service, Temecula,