Wow, there are a lot of issues in here.
1) when you say the power control task doesn't work, that may be a known issue. Call it in. HOWEVER, you might try the reboot to task instead.
2) if you have an image with an old agent in it (e.g. captured in 7.1) and you attempt to use the Reboot To task to get back into production, it will fail. Let me repeate, it WILL fail. This is because the task does not actually complete until it gets into production and receives the task AGAIN from the task server. At this point, an older agent version will not understand this rebuilt task and fail it because it doesn't know how to execute it.
2a) as a subnote, IF your production image is lacking the deployment agent, it will also fail.
3) the forced PXE mode and/or IP helper situation has absolutely nothing to do with whether or not the task is received. That may be a common factor in the environment, but trust me, it has NOTHING to do with the error you're troubleshooting. You'll need to look at the Task Server in that environment instead, not the PXE configuration. If you need to proove this, take a thumb drive with the PreOS environment on it, and test in both environments. You'll get the same results.
3a) UNLESS it's the preboot environment itself that is damaged in one of those places. The thumb drive may root this issue out as well. If the thumb drive image works in location A but not in location B, then you've learned something, right? Not definitive, but it's a start.
So, everyone, take a look at YOUR situation, and let's get some answers!! :D