After you write 20h to the display it is set up for 4-bit operation. From this point on the display expects to see two ENABLE cycles for each operation. Your Check Busy routine only does this once per access. When the busy lo is detected you have to provide one more Enable cycle. On your next acess to the LCD you are sending an upper nybble when the LCD is expecting a lower nybble. You must be very careful in 4-bit mode that you always provide two Enable cycles for every operation, including checking the busy flag.

Hal