Richie
There are 3 flap valves on the system
2 inside the pump and one at the strainer located at the entrance of the water from the bilge. 
When you leave the system dry for a while the flap valves deform and allow air to flow back reducing the ability of priming.  I had exactly the same problem when i replace the bonding stripe.  The solution is simple:  take the tube out of the bilge and place the strainer above the level of the pump.
With a hose fill in the tube though the inverted  strainer. Making sure you have enough water in the bilge  , rapidly place the tube back on the bilge and turn the pump on
water makes the valves work much better than air.  Make sure that the strainer is still covered with water at the end of the pumping cycle and if not adjust the line that turns the pump off.  If the strainer is not submerged air will slowly enters and causes the problem again.  Also, if you use the manual bilge pump, do not pump more water than that leaving the strainer under water   

Good luck

José Venegas
Ipanema SM 2K 278
Currently in San Andrés Colombia 🇨🇴