Persuading players to learn/unlearn PPM's?

When a player doesn't want to learn/unlearn a PPM, how am I supposed to go about persuading them into it?

It seems no matter how hard you try it is very rare for a player to actually give in to your request, it's extremely frustrating. I tell them I have greater knowledge, they ignore me, I tell them I'd like them to try it, they ignore me..

So my question is, is there a best way to go about persuading them into learning/unlearning or is it just rare to find a player accepting your request? Also is this how it should be? Or should it be easier for us to tell players to do it?

Edit: I'd also like it if a player refuses to learn the PPM you don't have to wait another few months to be able to request him to learn something again, time is short for players in FM.