Karl has said near enough the same as what I was going to say.
Check the mAh of the batteries in your lights. Low mAh means that they don't hold much charge - the higher, the better. I'm guessing that they'll be quite low, prob around 700mAh. Ideally you'll need 2000+.
Also the days are getting shorter, which means less sunlight hours, plus the sun isn't as strong. This means the charging time is less and the lights will be on longer whilst it's dark. I'm assuming that they're like ours and simply come on when it goes dark and switch off when the sun comes up??
We have a solar powered door number on the wall.
I've noticed that the days are getting shorter recently, because in the summer it usually stays lit all night; but I recently noticed that it now comes on in the evening but never lasts all night. It runs off one 2100 mAh battery, but there's just not enough daylight to fully charge it ......