Many times the FLASH programming process requires a lengthy period of time to cause a byte to be programmed. This could be say 1 or more milliseconds. If this is the case for your part then it is necessary for you wait for the byte write completion before moving on to the next byte. How you do this is very device specific so you most study the device datasheet carefully and any supporting App Notes and samples supplied by the device manufacturer.

On some types of FLASH based MCU the wait time is automatic in that as the IAP code starts to write the byte the hardware in the MCU holds the program execution until the byte write is complete.

Michael Karas