Unless your client conditioned payment to you upon their receipt of payment from their client, they have no right to withhold payment from you.Your contract is separate from the contract they have concluded with their client.  Payment is due within the time period to which you both agreed up front. If no specific payment date was agreed , then it is due within a reasonable time after you rendered services...30 days is certainly a reasonable time period. Delinquent clients just play the odds that you won't go to the trouble of going after them for payment. Unfortunately,  most of the time they get away with it.