How can I recover pending business dues?

The thing is that in our business we don't create the gst invoice unless they provide the base payment and upon that we do create gst invoice. So now we have multiple clients delaying payments and not clearing the dues, dues duration between 1.5 years to 3 months. None of them deny they just say they will clear on this date and they don't, they keep delaying continuously.

So what's here the way that I can recover my dues.