Baby has ‘forgotten’ how to latch

Ch

I have a 3 week old son and in the last few hours it seems as though he’s forgotten how to latch onto my left breast. He will latch and feed but he keeps slipping off; he’s fine on the right breast.

I’ve tried our usual positions and I’m just unsure why he isn’t latching properly because we’ve not had any issues previously. My breasts aren’t full/engorged as I know this can alter the shape of your nipple and confuse baby. Any ideas why? Thanks!