2nd hamster died in short space of time

[Blondie160]

I have the mini duna cage(for a chinese/russian dwarf). Ive had 2 hamsters in it (seperately) and both have died within about 3 months of being in it. Both have taken to crawling upside down along the metal bars, and falling off quite a lot. I just found the 2nd one dead and feel guilty that maybe its not suitable at all as they fall and hit their heads, has anyone else had similar issues? All my other hamsters have died of old age and this has really upset me.