Rechargeable cells provide 1.2V, rather than the 1.5V from conventional batteries. So you're already starting with a lower voltage when you use them.
Further, ALL rechargeable batteries suffer from the 'memory effect' whereby they won't charge fully unless they were fully discharged first. (I used to believe, as many people do, that NiMH batteries don't suffer from the 'memory effect', unlike the older NiCd batteries. But then I experienced similar problems to yours and emailed Duracell Customer Services for a response. They confirmed that NiMH cells DO suffer from the 'memory effect' but simply not to the same extent as NiCd batteries).
I gave up using rechargeable batteries in my camera ages ago and switched to Lithium cells. They're not cheap (but watch out for special offers in places like Maplins) but they last for ages.
Chris