A few months ago we bought a used hot tub that had a few leaks, and used it until one of the leaks got bad enough to drain 2 feet of water in one day. Since then I replaced most of the components; pump and wet side, blower, manifold, heater element, and pressure switch. I am not sure of the make of the tub, but it uses a Brett Aqualine EM-501 controller and Brett aqualine heater. My problem is that the heater will not come on during low pump, but will during high pump. It actually heats fine when I have the high pump on, but I can't get it to come on automatically on the low pump.

The tub is wired for 240.

What am I missing. Did I wire the pump incorrectly?

It is possible that while you have it wired for 240V the system may think you are at 120V, therefore the system would only heat when the low pump is activated, in addition to that you may have wired your new pump backwards (low = high, high = low) check to see if the timer controls the pump on high speed instead of low speed, if true then swap the black and red wires on the pump, also check your wiring diagram as there should be a plug inside that you can tell the system that you are in fact hooked up to 240V.
If none of the above is true then you may just have a faulty pressure switch, try running the system with no filter to see if it then heats on low speed, if so clean the filter and replace. If that does not work then replace the pressure switch altogether.
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Top Tech, pardon my naivety but how does a bad pressure switch allow heat on the high pump, but not on the low pump mode?

A pressure switch may not be bad altogether, it be out of calibration or it simply become "stiff", when this happens it requires more pressure to activate than the low-speed pump has to offer. Therefore it would work with the higher pressure that the high speed has to offer, remember that if a pressure switch is open a heater WILL NOT activate, that is of course the sole reason for having a pressure switch.