For most switches I just loop between the leads to activate/deactivate them. Later today I will go look at the tractor and see what/if the seat switch is done.
What I am describing is the shift is the switch activated by the Hi/Low lever. On this switch look at the activating linkage to see if it is binding. That is what usually happens with mine when I mowed in heavy/dirty fields. It used to happen to me later in the year when growth was matured, more seed and drier.
One of my PTO switches hung up a couple of weeks ago, and I just moved them several times to clear whatever was obstructing it. Don't know which it was.