The problem is that the SoftwareSerial library has to simulate this serial port using processor time, and if you try to use a baud rate that is too fast, the slow processor and limited memory of the UNO can’t keep up. This means that it’s not recommended to use baud rates faster than 9600 for the software serial port.
Of course, as I said earlier, the SIM900 and the UNO have to be talking to each other at the same speed, so you’ll need to tell the SIM900 to use the baud rate of 9600 as well.
You’ll need to google how to do this, via an AT command.