There's two possible explanations:
1. Protein converts to glucose with 58% efficiency. You're eating too much of it, and being bumped out of ketosis.
2. Your method of measuring ketosis is flawed. Most people use ketostix, which only indicates ketonuria (ketones in pee), not ketonemia (ketones in blood). Your urine could be too dilute for you to register ketosis on the sticks.
You can buy all sorts of fancy gadgets to tell you what's going on internally when you diet (blood ketometer, blood glucometer, ketostix, jelly doughnuts, etc), but what matters most is whether you're steadily dropping bodyfat. If you are, don't sweat the rest.