When you say the trifoam isn't coming on until the middle of the car, are you talking about the water, the chemicals, or both? The trifoam cycles all begin at the front of the vehicle on a Softgloss (photoeyes are not involved in starting trifoam).
If water is not coming on, with trifoam detergent, this is controlled by valve V41. With trifoam wax, it would be valve V43. After each of these valves, there should be a check valve. If the check valve has debris or has failed, the water can drain back from the line and it would take some time to recharge.
If you are getting water but not chemical, I would suspect the check valves on top and bottom of each of the chemical pumps. It's unlikely that all three colors would be affected, but look at the check valves for each one not working.