Well, Maeve hasn't been the Lady for as long as Mab was Queen, so there may not have been one at all. Or, this may have possibly predated Mab being Queen. OR she may not yet have trusted Lea, as she didn't yet have the chain of obligations to control her.
OR
The duties of each queen are disparate enough that one may not necessarily be able to act as another in all ways. For example (maybe/probably) the Lady might not have been able to have sex with the Winter Knight the way Mab did.