There can be multiple reasons for non-payment of GST amount by buyer viz. he is not able to avail the ITC because it is not reflecting in its GSTR-2B, he has such policy to pay GST amount only after a certain period, he might have some issues with your invoicing pattern or supplied goods or service etc. etc.
Reason for non-payment should be identified by you after discussion with your buyer. If he quote any legal position, the same you can share here for which we will help.