Kneel for the guidance of SIM900A and STM32.

Have you linked the sending of computer with the receiving of STM32, and the sending of colleague SIM900? In this case, stm32 cannot receive data. (The level will be pulled high in unison)

Or if there is something wrong with your program's serial port receiving interrupt setting, determine whether the interrupt vector setting is correct.

It is best to provide the circuit diagram and code, otherwise it will be difficult to locate.