It has happened to me few times that battery was so much dischaged that Fuze+ seemed to be dead. Even if connected to USB charger it switched on for few seconds (to bootloader) and switched off. I thought, connector is bad or charger is bad, etc. Or Fuze+ is dead.
My experience is that one may revive Fuze+ by starting Fuze+ in OF (by holding volume- while pluggin to USB charger) and chargé it a bit. After a while (few minutes) one may switch to RockBox and charging works again.
This problém has occured only seldomly (cca 5 times in 2 years), when Fuze+ was deeply discharged.
My guess is, that RockBox bootloader starts charging too late (the battery is not able to feed RB bootloader to the point, when charging starts). If my guess is right, and it is possible to switch on charging sooner, it might help.
I do not consider it a real problém if one knows what to do (switch to OF), if Fuze+ does not want start charging in RB.