It's well known that many cheap RF modules, particularly AM types, have difficulties to transmit UART data reliably. If you read the datasheets thorougly, you'll find that they specify a modulation duty cycle of e.g. 30-70% that can be easily achieved with a HT12 encoder, but not by arbitrary UART data.
Some RF module suppliers are also linking Arduino code libraries that are encoding serial data differently to get more reliable transmission. There are various previous discussions at Edaboard, e.g. suggesting a kind of pseudo Manchester encoding transmitted over standard UARTs. I don't want to retell the whole discussion, just mention their existence.