Strings and Loops inside AOI

I'm having a problem with an AOI I have created from standard ladder logic.

The Ladder code works fine outside of the AOI.

The code is manipulating strings and also pulling them from a 1 dimensional array.

It generates a message header (STRING) , then pulls the relevant message (STRING) from an array and concatenates them.

This is done 50 times, using a JMP LBL loop.

Are there some limitations of loops/timing within an AOI that may be affecting the code execution?

I believe I read about this a long time ago, but can't find any reference.

PLC is an L72S

I have one last thing to check before I am lost for options.