Do this,

You said one pop bumper works fine, so leave that one alone. You said the other ones activate immediately.

I would check simple stuff first like the leaf switches under each pop bumper making sure they are not closed from the start.

If that looks okay I would triple check the diodes are correct.

Then, yes I would check the transistors for those other two pops. Someone will post a link to Clays guide here shortly I am sure. I dont have it handy.