Oblivion has a number of NPCs who can charge weapons, for example Ita Rienus. However, sometimes when I talk to them the recharge option is not available. Why is this?
All merchants and NPCs that offer services have specific times, during only which, they will be offering their wares or services. As mentioned by yx, in the case of Ita Rienus, her available hours are from 6am - 6pm.
You could use the wait function until you reach the time that Ita Rienus offers her services.
At the UESP wiki, the time NPCs will be available to offer their services are shown at the upper right panel of the page, at the 'Services - Available' section.