When you say the main box is on is it in standby or actually on and in use. If it's in standby both boxes need to be set to Fast start or Active start in power settings so the mini can activate the main box and retrieve recordings. Is your mini connected via wifi and is on demand and catchup OK.
It happened to me too yesterday evening. Both my boxes (upgraded V6s) are set to 'Fast Start' yet the on-screen message box had the warning that my second box - the one I was trying to record to - was offline. The recording could not be set. Not a big deal as I set the recording on the box I was using at the time. My boxes have just received the 4.32 software update and this is the first time I have received this warning. I wonder if this is a bug that has crept into the update.
Following on from my previous post, yesterday evening I attempted to set a recording on my bedroom box but was informed via an on-screen box that my living room box was offline and setting the recording failed. I then set the recording on my bedroom box. When I tried to review it this morning on my living room box it stated that it was a planned recording even though it was in my saved recordings section. I went to my bedroom box and the recording was in the saved recordings and is playing back as normal. I re-booted both boxes and everything was back to normal. This has only happened since the 4.32 update. A new bug?