I also do lift off process on no-doped GaN but I think it should be the same
thing.
To answer your question, I think you don't put enought resist on the sample
before spin coating.
I had the same problem, there is resist only on the center of the sample.
If you have tiny samples, like me ( 5mm*5mm)
Try to do that : put your sample on something else like a glass wafer or a
silicon one or something else just to stick on it.
use your resist as a glue, post baking some second.
Spin coat, but put enought resist to overcover your sample and a part of
your support wafer.
It's not sure it works but I do that and the lift off works
An other thing, be sure of the thickness of the resist layer
PS: for information my procedure is
1) cleaning wafer like you
2)spin coating a positive photoresist (S1818) at 3000 rpm for 10s
3) post bake for 2 min at 110°C
4) exposure
5)develop with MF 319 30s