OpenCL error on code compilation

Thank you for your explanation and happy new year!

I am trying to understand where is exactly the bottleneck, thus I enable the ACL_PROFILE_TIMER variable to see the memory transfer.

It seems that the access to the global memory does not reach 100% occupancy but only 4.2%. Moreover, in the kernel execution panel, there are empty spaces that represent the global memory access time, if I understood correctly from the OpenCL best practices guide. I have also tried with 128 iterations of 64 bits each of transfer from/to global memory, but I did not see improvement. Please find attached the screenshots from the profiler.

How can I improve the occupancy?

Thank you!