Well I tried the new transformer and it lights about half the sign up.
guess I’ll look for a 5k
I know this thread is old, but even a transformer without enough power ( overloading the transformer) will light a small neon sign.
If it's only lighting half the sign, it likely a bad neon unit or bad electrode. You will need to make a pattern & have a neon shop repair or replace the unit.
Even a hairline crack will allow the gas to escape & the circuit will not be complete.
If your working on neon you need a neon/ florescent lamp tester. It allows you to test light the individual neon units ( tubes) just by touching them.
There are some very inexpensive ones available.
Under loading a transformer is when the transformer is to large for the load. not as bad as overloading the transformer.