That's a weird one as you have what seems to be a new drain point soldered onto another old drain point there. They can get blocked by sediment/sludge, get a piece of wire say from a length of 2+e cable and see if you can clear it.
Only other way is to unscrew the newer drain valve all the way out and back in again if that is the one that's blocked, just be careful though as you may have a lot of water when the blockage frees up. The outside, knurled ring will unscrew allowing the centre valve to be fully removed.
Only other way is to unscrew the newer drain valve all the way out and back in again if that is the one that's blocked, just be careful though as you may have a lot of water when the blockage frees up. The outside, knurled ring will unscrew allowing the centre valve to be fully removed.