I've yet to get one of these for myself, so I have to apologize if this comment is off the mark. However, "solar powered" implies that there's a way to store that power for night use and that would require rechargeable batteries, not just any AA. Something like this:
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or these:
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because some of the solar light umbrellas descriptions do mention AAA rather than AA batteries. It's possible that the unit requires both types of batteries although it's rather unlikely. I would switch to rechargeables and see if it fixes the issue. The links I provided are to high capacity versions - I always replace "normal" rechargeable batteries with high capacity in my solar lights.
If you have more information that would help with a more detailed answer, please post it in the comments.