HC-06 Bluetooth module and slow communication
There are a few things that may be causing your issues:


The HC-06 may be a cheap knock off and may not perform to the level that you expect.
There could be an intermittent wiring fault or loose connection.
The signal strength may be causing your issue - it just can't 'hear' your data.
The antenna on the HC-06 module might be compromised in some way resulting in the loss of signal.
The data rate (baud rate) may be too high.
Your code may have blocking functions or delays that is causing you to miss the data.
The Android app may have bugs in it.
The Arduino program may have bugs in it.