I found that when flying from one country to another the chart would not load for the upcoming country, even when flying through it! I just ended up flying through blue until I returned to the first country! The charts are all installed on the sd card. 

That's not normal behaviour!

Just a thought, does anyone know if it is the land border or the FIR that triggers the next chart upload? Sometimes the land & FIR borders do not coincide!

Just a thought, does anyone know if it is the land border or the FIR that triggers the next chart upload? Sometimes the land & FIR borders do not coincide!

It's usually the FIR border with some notable exceptions, for example if you're within the channel islands class A that's considered to be both UK and French chart coverage. The chart load is triggered when you're about 10 minutes from the border of chart coverage.

So, where FIRs & land borders don't line up it may be necessary to force load a chart?

It should never be necessary to force-load a chart during normal flying. If you give me an example of where you mean, I can give you a definite answer.

For example a flight from FLND to FLMA?

For our African charts the FIR boundary will be where charts are switched.