This can happen for a couple of reasons. If this happens with just one client (the clients before and the clients after had normal results in the water) then it is very possible that particular client did not have a great release. This can be due to stress, anxiety, lack of energy, etc. It is important for this client to continue with the footbath as they will experience an incredible release in the next couple of sessions.
Follow the steps in this video to remedy the issue.