Bootcamp assistant won't let me increase the Windows partition
I think alimack is right, has to do with contiguous space.  I resolved without purchasing idefrag.   Heres how.  In my case I had a 250gb drive and was only using about 50gb.   Boot camp was only allowing me to create a 125gb partition for windows.   I accepted that and partitioned the drive in boot camp. Before installing windows I rebooted the Mac OS, went into Disk Utilities and then deleted the windows partition.  Now start again and run bootcamp and you will find a lot more space is available for your windows partition.