New Dimmer Switches Work Inconsistently
Since the problem started when you added the transformer, that is the first thing to check. Three possibilities:

You somehow wired it into the circuit in a way that it ended up in series with the dimmers/lights. That would be rather unusual, and with incandescent lights you'd probably have no lights at all. But LEDs don't need much power, so you just might end up with a &quot;sometimes works&quot; situation as you described.
Bad connections. If you spliced in hot &amp; neutral for the transformer so that the lighting part of the circuit is on the same wire nut (or other splice) then a bad connection could cause high resistance and strange problems.
An actual interaction between the transformer and the dimmer/LEDs. Unlikely, as a transformer is a pretty simple device.

Disconnect the Transformer
Disconnect the transformer and see what happens to the lights. If that fixes the problem then you need to try:

Make sure the transformer is NOT in series. If you are not sure, post pictures of the wires in the boxes for the transformer and the dimmers.
Reconnect the transformer, checking connections carefully. If you use wire nuts, get a new set of wire nuts and make sure they are the right size for the number &amp; size of wires.

And if all else fails, move the transformer to a different circuit.
If removing the transformer does NOT fix the problem, check/redo all the connections for the dimmers &amp; lights. If that doesn't solve the problem, replace the dimmers with regular switches and see if the problem goes away - that will tell you if the problem is the dimmers or the LEDs.