There are some Yami models that have a manufacturing defect in the throttle bushing. You can search the web for Yamaha outboard recalls and see if your motor is one of those models.
I just checked and the F150 was not included in the recall. It was 2003,4 and 5 F200 though 250s
Note that tight throttle cables will also make the shifters hard to operate. Use the shift override button and see if the throttle operate OK.
Is the shifting problem getting into neutral only or both in and out of gear? As asked, are the motors running when the problem occurs?