run time -- some questions

Dear all, 

I have written a very long code, with many subroutines.

In this moment I am runnung some simple run-time analysis with 

   CALL CPU_TIME()

I have notice something strange. When I increase the number of cycle in may code the running time increases not linearly. 

I mean if I do four cycles instead of two, the running time does not double but is is greater. 

Now, I am looking to some convernce of my code. However, I would like if there are some possible other reasons to check in my code. For example some problem linked to subroutine call.

Thanks a lot