Hello Debolina,
You are facing the issue because, Scheduler is not running, or else there is a communication or resource problem.
If your queue(s) are unregistered, you can register them using SMQS. Look for a missing destination or a destination with a 'U' (unregistered) in the first column.
Check if the queue is waiting any queue to be processed. Generally dependent queue entry is visible in the last column in SMQ1 transaction.
Worst case you can write an report to process queues, upto 30k can be processed in a day depending on the data content in the BDOCs.
I hope above reply is helpful to you.
Regards,
Kapil