Delayed payments from vendors

Topic: Collection of payments from clearing and forwarding agents. The collection of payments from these agents is a tedious process involving months of credit period and rigourous follow ups. The payments are received after 6 months and sometimes even 18 months. I have 2 questions; 1) In such cases would vendors be forced to pay the gst tax? 2) or if the service recepient pays only the gst tax can he continue enjoying the credit period as usual for more than 3 months.