It really depends. It's very easy to feel as if someone is being evasive, for example, but it's impossible to know for certain without being aware of what their baseline is. Shy people, in particular, can feel intimidated by looking someone directly in the eye, whereas a sociopath can look a person squarely in the face and not think twice of lying to them. It's a difficult one without knowing the context... Sorry if that's of little help, but I rarely rely on body language alone, as I tend to go with gut feel. 

I agree with Justin. It could mean a lot of things...

It could mean you don't have their full attention - and not necessarily because of you or your question, but just that there may be something else on their mind that keeps pulling them away. 

I'm not particularly great at eye contact. I've consciously tried to improve it since I realized it doesn't always come naturally for me, but then it feels awkward and forced when I am thinking about doing it just to be polite. Sometimes I'll be chatting with someone, going through small talk, listening to their story, and I really can't focus, and I feel trapped. Then when the conversation lands somewhere that genuinely interests me, my eyes fall into place and it engages a whole person connection, and then I feel like I can breathe. Of course I have a pile of Gemini-Sagittarius oppositions.

depends -

evasion?

autism?

distraction?

I find eye contact to be a distraction while I'm trying to explain something complex. My focus is on trying to get the answer out.

I wouldn't read into this either way.

I tend to think it means they are hiding or trying to conceal or suppress something .... Like part of their thoughts or feelings. Shyness/intimidation, like, dislike, their intentions... You never know.