I'd cut the pipe down to less than the height of the washer if its feasible.
The pump may not drive all the soapy water 6+feet uphill, Allowing it to run
back down back into the machine once the pumping action stops. Then try half
the soap indicated on the measuring cup device

Just got a new machine (Sears Kenmore) at a house I just bought, but even
a minimal amount of soap won't seem to drain. The pipe going to the
septic system is a little high (about 6 1/2 feet from the floor) - could
that be the problem?