She could just really know herself well and is just saying once a week thats it. I mean its not like she saying no shes just saying once a week and not everytime I see you. If she was geting it somewhere else I don't think she even be doing you once I week but scheams can be complicated. Was there ever a time when she did more. This seems to me like 2 diffeent sex drives. You have the right to ask for more info from her as to why, if shes unwilling to say or can't talk about it than maybe somethings up, but if shes always be like this then you cant tell her to change and get more into it.