Does a union have to support an unfair dismissal case if I was a paid member at the time of being dismissed?

I was unfairly dismissed and was a paid up union member at the time. I subsequently missed a payment and despite offering to bring payments up to date am being told by my union rep that they are refusing to now support me on that basis. Is there anything I can do about this as i couldnt afford to fight the unfair dismissal case without the unions help.