Parallelization only part-time

[Optiuse]

Hello,

when I want multithreaded parallel function evaluations for 8 Threads i do this:
#define OPTIM_USE_OMP
omp_set_dynamic(false); // omp setting
omp_set_num_threads(8); // omp setting

I have very expensive functions and some parameters take much longer than others. The problem is that often not all threads are used. Overall, this makes the optimization much longer because the CPU is often only used to 1/8 of its capacity. Therefore the effect of parallelization is only part-time.
It gives the impression that the parallelization is interrupted as soon as a "slow function execution" occurs in the overall process. As if the execution of new threads would have to wait.

What is the reason for this and is there a possibility that all 8 threads are always used and therefore the cpu is used at full capacity? This would significantly accelerate the total optimisation process.

Greetings