Jane - is that the cat Metacam or dog? the cat Metacam is 3 drops per kilo of weight, so am surprised one drop is making a difference.
I would try heat beds and other glucosamine products first - our locum vet told me it didn't matter if I gave Zi a 1kg or 3kg dose, it doesnt' make any difference if it is going to affect their kidneys (she developed CRF after being put on daily Metacam, but we cant' get her back off it). It might be worth asking about Onsior, it is supposed to be kinder to the kidneys than Metacam, it is only licenced for 6 days, but if you sign a disclaimer, you can use it on a 6 day on, 6 day off basis (my vet rang the manufacturers when we tried it on Zi). If she is like Zi and wont be pilled, and she ends up on Metacam, please do get regular (minimum 6 monthly) bloods to check her kidneys, and if she starts drinking extra water, vets asap (Zi did within days of being on Metacam, luckily Renal food has drastically reduced her kidney levels) - it can also cause stomach issues, so watch her poos as well, any blood is an immediate call to the vets.