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A big welcome from me too @Crakin!
I'd agree with @Marty_greenshed that it sounds like you are getting a fairly limited runtime. Remember, though, that you've still cut through around sixty 137x23mm boards. Either there's another variable involved here, or the battery or, potentially the saw is faulty. You weren't cutting into the frame underneath the whole way along, were you? Was the battery reading that it was completely drained, or was it overheated? If overheated, it would start working again when it cooled. The DSS611Z is designed for light trimming work, and cutting a continuous 23mm thick board (despite the gaps in between) over 9m would likely be considered at its upper limits. The battery will become overheated when the tool is overloaded.
Also, could you tell us more about this blade that's specifically for composite boards?
Please let me know if you have any questions.
Mitchell
It is not an issue with hardware compatibility @Crakin.
The battery heating up indicates that the tool is being worked reasonably hard. There's no issue in doing so, as the battery has a thermal overload that will cut power if pushed too far. It sounds like the overload might have tripped when you were on the last bar, and there was potentially a bit more juice left in it. If so, it would run again when cooled down.
I've had a chat with the Makita tech team, and we are both in agreeance that there potentially is an issue, though it's more likely that the nine-metre stretch of relatively dense composite decking has just drained the battery. I run mainly Ryobi tools, and if I drained my 5.0ah battery over a nine-metre cut, it wouldn't surprise me at all. Because you've drained the battery over one long cut, it does make it look like it's not cutting much, but if you think of it as you've cut 137 boards on a single charge, that's actually quite a lot and could cover a whole day's work for some people.
We are more than happy to send the battery off to Makita for you so they can run some diagnostic tests on it. If you see our helpful team in-store with your proof of purchase, they will facilitate the warranty assessment.
Mitchell
I used a chalk line and clamped a 5.4m comp board. I didn’t buy a new blade for the ryobi and the new one for the makita is not compatible so it could be that, but the ryobi blade tended to move itself off the path. Not the best discription I know, but whether it was the old blade, me pushing too hard, the blades tendency to slightly bend with the toughness of the composite boards or an combination of all the above im not sure.
In retrospect I overestimated the power of the makita and probably should have invested in a decent blade for the ryobi specifically for composite boards. This was the first deck I had built, from tearing down the old one to rebuilding from scratch the new one I felt I had spent more than enough 
the makita worked really well with the new blade, just only for minutes. I got there in the end, and as usual, a lot of lessons learned
just the Balustrade to do
Hi @Crakin
Thank you so much for posting your low-level decking project. It looks awesome, the colour you've chosen for your decking boards is very nice and blends in perfectly with your wall colour.
I've run into a similar situation where the saw blade begins to drift out of line. Sometimes you've got to be vigilant to make sure that the cut is straight and true. It definitely helps if you have a premium saw blade on your circular saw. Even an ordinary saw can produce outstanding results with a quality saw blade.
Please keep us updated with your progress, we look forward to seeing the balustrade installed.
Eric
