I guess no-one else wants to tackle this one :)

In my mind it comes down to issues of self-worthiness and self-esteem, probably originating in guilt-based experiences from one's past.

If one is feeling awkward in receiving compliments, it implies that one doesn't feel worthy or deserving within oneself of being complimented.

If one is having a hard time in giving compliments, it again implies self-worthiness issues in that giving a compliment to another implies that one is somehow demeaning oneself in doing so. Alternatively, it can imply that another doesn't deserve to be complimented...a similar judgement-based idea to the one that says that some people don't deserve to be wealthy, again stemming from self-worthiness (self-esteem) issues.

I'm not implying that any of this relates to you personally, of course - just stating my own observations of this phenomenon :)

I used to have the same problem. I realized that it made me feel vulnerable to give a compliment. I suspect other people feel the same way. When we give a compliment we show a part of ourselves that is soft and loving. Nobody wants that to be rejected or discounted. That's why it is frustrating with someone brushes off a compliment. You're trying to give them a gift and they refuse it.