Are you multicasting ? If you are, have you got IGMP snooping enabled on your switches?
What operating system are you imaging? If windows 7, there are two partitions that need to be imaged in order for Win 7 to boot - that's ignoring any vendor partitions. The partition numbering is also a vital component in this equation, so if the partitions are arranged differently on the target machine, you are going to have problems.
Basically, we need a lot more detail about what you are doing and how you are doing it. If you are using a boot image, is it DOS or WinPE?