The issue is that the clear sections at the back of the flow frames need to be pressed up firmly against each other… this provides the ‘windows’ or ‘galleries’ when you remove the rear cover. If you have a standard Langstroth frame in between two flow frames this back section won’t be sealed off when you remove the rear inspection cover.
There are photos of this exact issue here:
http://forum.honeyflow.com/t/my-hybrid-hive-plastic-frames-and-flow-frames/8571