You said that a while back you used to do a lot more romantic and intimate things for her and you cut back on a lot of that because you didn't feel it was reciprocated. We have another member on these boards who also gets sex once every couple of months because his wife is feeling a lack of intimacy. Maybe when you started to cut back on the more intimate stuff, her disinclination towards sex became a type of passive ressistance. I think looking at the situation from this point of view will help to see how the past occurance of action and reaction have led to this problem. It sounds like the two of you need to seat down and talk. You need to tell her that lack of sex is an issue for you and you would like to find a way to resolve this. Bring a number of methods you would like to try for this to succeed in to the conversation (like being more intimate and romantic) and see what her reaction is.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~