What are my options if I couldn’t get the money that was owed to me for the time I already put in?

I have recently planned on leaving old job because of the low pay and on top of that they always hold the raise that we are supposed to get for about 8-12 months which causes us to get a back pay check when the raise does happen. The same week, I was going to leave. They gave out retro check to everyone that was still working with the company. I texted my area manager asking him why I didn’t get a check and he told me it was because I was not in the system anymore. This was shocking for him to tell me.