The Priority field and WBS on an Activty seems to have no effect in allocating the time available

I required an Activity to take priority over another Activity, however the Priority field has no effect what-so-ever. The standard Priority level is 500 I tried using 100 as a lower Priority number should have given the Activity preference over other. Nothing was changed.

I also read that a lower WBS number should also give priority. This too had no affect.

No dependencies between tasks
A resource was assigned to tasks and had no time left
This was the reason that using a higher priority should have overridden the planning.
Resource had no spare capacity.

Therefore, my question is if using a higher priority has no effect then how can it be used?

The screen print is of no use as the planning of that task does not move, not even within the project iteself.

Similarly once a task has been planned then moving the WBS of the project up from the bottom to the top and then using the calculating activity planning has no effect.