Without a scope its a guessing game where the issue is.
Even one of those cheap 1MSPS $20 scopes would help track this down.
I expect this circuit to be a lot more likely to blow the mosfets due to overcurrent into the transformer than to run with a very low output power.
What are you using to power the circuit and are you sure it can handle the high surge current that this thing will draw?
Maybe try something with a bit more grunt. RC lipo cell, SLA, car battery etc..
Just be prepared for a bang and mosfet chunks to ricochet around the room.
eg safety glasses
Try set your DMM to min/max mode, connect it across the 12V input then run your circuit.
See what the min voltage is.
One other question, as i have seen people make this mistake before.
Do you have these 4 way joins connected correctly on your circuit.
Sometimes people miss-interpret them as 2x two way joins passing over each other because they do not notice the dot mark.
See image below.