Concurrents are running for long time and not getting completed

Hi
Could you please share me your advice on the following issue.

On R11.5.10.2 environment.the processes for standard manager is 30.
when submit 145 concurrent requests(eg:request A) ,It shows 30 concurrents are in running status,
others are in pending status,but 30 concurrents are running for long time and not getting completed.
As a result,it increased pending requests
(Note:request A don't have any Incompatible Programs)

As a workaround,we cancelled all the running requests,let the requests which are in pending
status get proceed firstly,then rerun the cancelled request.

Could you share me your opinion on what is the possible reason that 30 concurrents are kept in running status and