Snap To Grid stopped working for me yesterday and I didn't understand it, so I searched and searched for answers without satisfaction. I found this thread and I started typing this message looking for help, but while documenting my issues, I stumbled on the answer
and I feel so dumb!! For me, the solution to the problem didn't have anything to do with magnetic strength being on or off (that feature can be helpful, IMHO) or being confused about the relationship of the main Snap To Grid function versus the special one in Piano Roll View. Nor did it have anything to do with those little buttons for "Snap To MIDI Landmarks", etc.
The solution was that I didn't realize I had accidentally activated the "Absolute Time Snap" button under the "Musical Time Snap" button. I mistakenly believed that it was normal for it to be turned on when "Snap To Grid" is activated, when in fact I was trying to use only the Musical Time Snap option.
So again, for anyone who suddenly finds that Snap To Grid no longer works for them, just make sure that when you activate Snap To Grid in the main Control Bar, you only allow for the option on the top right to be active also: the one for "Musical Time Snap =". Later, you might decide to use the other options below the top option, but at that time, you'll probably study up on those options and use them on purpose rather than accidentally & mindlessly like me.
I hope this helps!
Cheers,
Allen