Even if the discharge is not everyday, the yellow color to it is indicating they still have an upper respiratory infection, and at this point may have developed a secondary bacterial infection in their sinuses. The clavamox is a great antibiotic for URI's but it sounds like they are resistant to it. The problem with these upper respiratory viruses is that once a cat has a bad case of one, they then tend to be "carriers" for life of that virus and in times of stress or if their vaccinations are not kept current, they will flare up with symptoms. I would take them back to the vet and see if you can get a different antibiotic...there are many others that can be good for the URI and bacterial infection. My favorite has been a new 2-week duration INJECTION antibiotic called "convenia"...it is a cephalexin family and has been working WONDERS for cats (it was created to help with abcesses in cats but has been great for UTI's and URI's too!) The other thing that I would make sure they are being vaccinated for is the new virulent systemic calicivirus - it is a very aggressive upper respiratory virus that has been moving through the country. I'm not saying that they have this, but they need to be vaccinated against it to make sure they dont get it. The immunity provided from the vaccination alone may help them fight this virus too. The other thing I want to mention is to make sure that they were tested for feline leukemia and feline aids (FIV) because with a weakened immune system, they will not be able to fight these viruses on their own. Hope this helps! :)
