Flushing problem

About 50% of the time the toilet does not flush completely. I have to hold the handle down all the way for it to completely flush, and sometimes that does not always work. Other times I have to use the plunger to make the water flush completely.
Not the most handy or DIY type person, so any suggestions would be most appreciated.

Thanks

Normally something left in the trapway preventing a good clean siphon.

A closet auger or snake may be needed to clear it.