Miller Home Fuel Oil forced hot air furnace keeps losing prime--Why?

You probably have a air leak in the oil line allowing the unit to loose its prime. Flare nut fittings,oil filter fittings etc are all areas of possible air leaks.

There can be other reasons like a bad pump internal check valve but usually that is not the case.

You will have to trace down your problems or check all connections to find the problem. It is not fun because you will not know if fixed until it stops losing its prime.

I am sure you keep your tank pretty full because a tank that is real low on oil can also be a problem.