That behavior could very well be normal - it depends on the batteries.
All NiMH cells "self-discharge" (though there are newer "low self-discharge" cells that do it much more slowly than in the past).
I've had AA NiMH cells that, after a full charge, would only hold that charge for 3 or 4 days while sitting on a shelf. These cells were purchased at least 10 years ago and have gone through an awful lot of charge/discharge cycles.
However, I have newer AA and AAA NiMH cells that seem to hold their charge for a couple of weeks, at least.