It could be lots of factors, it could be the A time, it could also be that your baby needs a bit more cuddle time, babies really do change through the day & as they get older the time may vary a bit throughout the day, some babies do better with a shorter first A time & some get more tired in the day & more stimulation means they need shorter A times later in the day.
The reason that A times are 2hours for 3-6months is that it's an average, babies and their environments are all different and so will their ability to cope... my first needed a lot more sleep in 24hours than my second, as my second is a Low sleep needs child. So it really is about working out a little bit of tweakage that suits your child.
The fact that you get the first 2 naps working well is really great, in all honesty it's pretty rare for babies to go down perfectly for all naps and bedtime, not because parents don't try, but because different days have different factors influencing them in terms of simple things from daylight, weather, people, noises, feeding etc, which is why looking for the tired signs is probably the most helpful thing, rather than exact times.
