If the cell signal is weak that could explain why the handover fails. It would probably work better to set Wi-Fi as the preferred method of communication.
Also, if your handset is off the cell network for more than 4 hours (I think that is the time frame) you're purged out of the system so when you go to roam from Wi-Fi back to the cell network the handover will most likely fail.
Something like that, I remember reading while working for T-Mobile that handover failures could be caused by being off the cell network for x amount of hours. But it's been a while and I've replaced a lot of that information in my brain with newer information since. :P