Hi there and welcome to BW
At this age it is pretty common for LOs to have developmental short naps. You may already know that they work out their night sleep before they work out their day sleep. With a good routine (which you have - good work mummy!) it is common for naps to lengthen at around 6 months.
At this age my LO had 4 or 5 naps of 40 min per day. At 5.5 months his first nap suddenly extended (he was born 13 days over so you could say he was 6 months adjusted!) and later his second nap improved too.
You can do as you are, trying to extend his nap, either all of them, or choose one to try on that one every day (the first one of the day is good), or you can accept that this is his routine at the moment and just get up and get on with the day. There are benefits to short naps as you feel less guilty about going out knowing that you aren't interrupting a good long sleep
I think you were right to try the longer A time, if it was very recent he might need some time to adjust to it.
Hope this helps