Hi, I'm not a spa technician but I've played around with spas for quite a while. I do have a couple of suggestions that you might look into to help. One. I've had the 'no heat problem' before and it turned out to be a water flow switch. Do not know if your particular unit has one but this switch is designed to keep the heater from turning on if there wasn't any water flow.
Along the same lines, no water flow could mean there is a problem with your pump. If you replaced the pressure sensor, this could contribute to the problem. Obviously, if the pump is not giving you high enough pressure the heater won't come on or if the pressure sensors pressure setting is too high, this might be the problem.
Another problem could be you have air in the lines, hense, no flow. I would open the water line just before the heater to let some water [and air if there is any] to see if you have 100% water. Hopefully, if you get some air out, things will start working.
On my tub control board, there is a LED the turns on when the system is calling for heat. Do you have one of these? This will help you localize the problem.
Hope these ideas help.
Let me know how it goes.