Skynet was losing the war. Sending back Terminators to try and change the outcome of the war was a last ditch effort. As such, Skynet probably couldn't afford to wait until it had developed a newer, more powerful terminator.
Not to mention, the T-1000 (Terminator 2) was an "advanced prototype" (according to the T-800/101), implying it hadn't even been tested in the field before being sent back. That is a sign of desperation in itself.
In T3, we learn that Skynet was delayed (not stopped entirely). But this also meant that Skynet started with more of a leg up than before. It had the time and the resources to develop better assassins (the T-X).
However this was a different Skynet to the one originally under development (Skynet 1 had a central control point, Skynet 2 was a distributed application in cyberspace). Why is this important? Because Skynet 2 didn't know about the earlier failed plans and therefore couldn't know to send back better Terminators for reinforcements. It originally sent back the T-X to take out John's lieutenants, as John was off the grid and couldn't be found. (It was only by pure luck that the T-X found John).
In conclusion, Skynet 1 didn't send reinforcements back because it was losing the war and either didn't have any, or lost control of the time-travel apeture too quickly, and Skynet 2 didn't send reinforcements back because it wasn't aware of the earlier assassination attempts, and couldn't find John anywhere as he was off the grid up until the events of T3.