How are you preparing the water for water changes? With the extreme cold weather that we are experiencing recently extra precautions against gas bubble disease need to be taken especially when doing 100% water changes. Laying over could indicate a problem with the swim bladder which is one of the organs most likely to be affected by GBD. If the discus are showing any stress immediately after a water change this should be considered the most likely cause.
In the past I've tried feeding beef heart to fry and experienced unexplained losses. Possibly due to bacteria blooms from the raw meat? Something to consider if the problem continues.