As mentioned above, as your pet levels, so must the level of your food or it will take ungodly amounts of chow to affect its level of happiness. I too almost alwasy keep my pet in the green for the bonus, so as I level I need to throw out the old food and get new. The trick to know if you are feeding it Primo food is go to the combat channal tab before you drop the food on him. The optimal happiness level for your pet is 35 happiness points per tic when feeding. If you are only getting 7 points per tic, you need to upgrade your food immediatly. To do so just kill higher level mobs for it, the higher the mob the higher the resulting meat will be (if you have a meat eater) currently I have a cat (meat and fish). Crabs are the best source of food I have found at low 20s. They drop meat very often (for fish eating pets only).
Also, if you are brand new and just got your pet and can not feed it, go back to your trainer and ensure you have the Feed Pet skill. If you do not, you need to train it or you will be on a neverending cycle of chasing down new pets. Hope this helps, good hunting.
Edited, Sat Nov 13 20:38:24 2004 by Dreadmortus