Population on PS2 has suffered over the past year, I've been playing it pretty much since launch (around November 2012) and it is clear that the population since then has dropped significantly - they've had to merge a bunch of servers to make up for this. So that's why it might be hard to find a big battle outside of prime-time for the server area.
Which server do you play on? Make sure you are playing on an EU server (Woodman, Miller, Ceres, Cobalt) if you are in Europe as the ping is pretty damn large if you play on a US server (around 200ms) which would definitely contribute to not so smooth gameplay. Unfortunately all the servers seem to have been downgraded since launch so even on EU servers (I play on Woodman) one gets pings of >100ms during prime-time (evening), which is not ideal of course.
But, still, despite the latency issues and the decreasing population, PS2 is still my number one played game as there is nothing else out there that comes close to the scale