why consumers can't receive message from the queue?

I use JmsTemplate with Spring on the consumer side, and set 600 ms time
out before consumers could get messages from queue, everything became
incredible every few hours.

There remains thousands of messages un-consumed in the queue from the web
console, but why consumers can't receive them ?

Is there something wrong with prefech?

every few hours, I need to open the web console, and purge the queue to
restore my consume, so please help me.

Thanks ahead.