I'm not sure but the usual problem with Pyracantha is Fireblight and it is a difficult one to deal with. There are some unusual treatments; one being Ferti-Lome, Fire Blight Spray but looks like only being available in the US. Perhaps dig it out and replace with something else. Anyone else have any ideas?

it seems to be closer to a wall than the other one so it may be getting less water. this would limit the growth. try giving it lots more water and when fully in leaf a good feed. I wouldn't get rid of it just yet.