You'll probably have to re create your live/boot pendrive, sounds like something has made it un bootable, either not set as a boot disk in the partitioner, or something has overwritten the mbr.
I have made many Live USB Fedora (F11-F15) bootable pendrive OSs but have always ultimately had to burn the Live iso to a CD first -
I for sure will do that again - if necessary, but now,
I have been trying and really would like to get a bootable (1GB) Fedora 17 KDE stick without first burning the iso to a CD
I have the F17 KDE Live iso downloaded and have made the UNetbootin executable just