APP INVENTOR Why can't Bluetooth read the value after sending some other commands? Only when the software is completely closed and reconnected, and no other operations can be performed before reading data, can data be read. Once other operations are performed, how can data be read?
We need a great deal of detail from you to help you ZSY. Please search the forum for "Bluetooth" - you may find your solution but if not, you will see the detail we need.
Define your code appropriately if your App needs to both send and receive. Think of a Bluetooth signal as water running through a pipe - it can't go in both directions at the same time (Increasing BLE MTU turns that pipe into a tunnel!).
ProfessorCad: Tips & Tricks Bluetooth
ProfessorCad: Tips & Tricks Bluetooth Failure