Had the same problem, then I looked carefully at the circuit and laughed my head off!

Like you I switched the POSITIVE side of the power....BAD MOVE..put the FET in the NEGATIVE rail so the circuit you want to power is effectively the LOAD use a pull down resistor of 10K or so and ALL will be well.

Took me a while to figure this out, but if you look at what is happening then it's obvious.

Hope this helps.