I've kept angels since the 80's and I can honestly tell you that unless they are eating or getting ready to spawn they are not overly active fish. When you have a few together they can even have very quiet/still times during the day. If you have a single angel then I wouldn't be too alarmed if it just kind of hangs around, that is pretty normal and since you say it actively eats and swims some I'm pretty sure it's fine. Some of my angels (I currently have 13 in a 220g) like to hang higher in the tank and some more mid level. Unless the angel is literally at the very top of the water level looking like it is trying to get more oxygen or is in distress then I wouldn't worry. Especially if you have other fish in the tank that are all fine.