One phrase, 3 meanings. Learn them one at a time, then test yourself.
Meaning 1 · most common
when a device, machine or part of the body keeps causing trouble by not working properly
Examples
The wifi's playing up again, the video keeps freezing.
My knee's been playing up since the weekend.
Common mistake
Means intermittent trouble, not a total breakdown — for that, say 'pack up'.
Meaning 2
To draw attention to something or make it seem more important or attractive than it really is.
Examples
The advert plays up the health benefits but says nothing about the sugar.
A: How should I prepare for the interview?
B: Play up your experience abroad — that's what makes you different.
Common mistake
Not: The ad plays up on the health benefits.
Say: The ad plays up the health benefits.
No 'on' — 'play up' takes the object directly.
Another example
I think some newspapers play up negative stories because they attract more readers.
Tell it apart
They played up the risks.
They played down the risks.
Meaning 3
If children play up, they behave badly, often to get attention (mainly British).
Examples
The kids always play up when they're tired.
A: You look exhausted!
B: My two have been playing up all afternoon — I've had enough.
Common mistake
Not: My son played up very bad at the party.
Say: My son played up at the party.
'Play up' already means 'behave badly' — you don't need 'bad' after it.
Another example
In my experience, children often play up when they feel they're not getting enough attention.
Tell it apart
The kids are playing up.
The printer's playing up.
Read the sentence, then pick the meaning that's in play.
Pick a meaning and say it out loud. Save the ones you mix up.