Silly question. Are you running more than one zone (solenoid) at the same time?
Most transformers are designed to output about 1 Amp and that really means not more than two solenoids (main + 1 zone or, 2 zones) and the RaspberryPI-OSPi.
If you have an AC ammeter, try measuring the total current taken from the transformer at its output. That reading may give you a clue. Also try the same measurement when you activate each solenoid in turn. They should each draw about the same current when switched on one at a time.