Hi @Khaled.Hassan ! Would you like to contact our support?
Can you give a bit more context (sequential/non-sequential, optimization / tolerancing, computer)? Can you share an example? Thank you!

This ticket has been solved by the support, but just as a side note here are a few helpful comments from my colleague @Ty Adair when encountering these kinds of issue.
“OpticStudio does attempt to use as many cores as possible up to its 64 core limit. However, optimization and raytracing can both be bottlenecked by a few factors: 

• The number of assigned variables during the optimization
  • OpticStudio will only use as many cores as the number of variables assigned
• The amount of RAM in your system
  • For each variable assigned, OpticStudio loads a copy of the entire system into memory before tracing/ evaluating.
    • Systems with imported CAD parts can commonly run out of RAM before utilizing all available cores.
  • To check the estimated memory required per copy of your system:

• Last, some Analyses/ Merit Function Operands can only be executed single-threaded
  • This could result in intermittent reduction in core usage during an optimization cycle.

 “