Sketch and Flow save each stroke and item as a separate object in RAM. After some time the ram fills up and it starts to slow down. The very easy fix for this is to save your progress, close the program in question, and then reopen it. If you have multiple instances open of the program close down each one and then open them back up. The more RAM you have the longer it will take for something like this to happen.
A second issue is that the Bluetooth receiver is occluded behind a wall or has another major source of electromagnetic interference. Mover the Bluetooth receiver to a location away from the source and so it is not obstructed by ferrous material.