Pump won't prime, recently plumbed in a heater

Hello, Ive recently plumbed in a heater and I keep losing prime on my pump. It'll run like normal but then after a while, it will struggle to fill up the watch glass and water won't flow. Ive tried sealing the o ring with pool grease and tightened the watch glass down. Still won't work. I can change the rpms of my pump with the Easy Touch on the side of my house. It says 875 rpms. Is my pump able to do this? Is it enough hp or rpms? Thanks. See attached pics.
 

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The pics you posted is of a single speed pump.

How can you change the RPMs of that pump?

Post pics of your equipment pad.
 
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