If you still have problems with them, try them on the PC again. By not connecting grounds between them it is possible that you blew one or more of the UART pins.
I was working with a co-op at one of my jobs one time. I had a MCU board with an analog input on it. I gave it to the co-op to measure an analog voltage. He came back and told me it didn't work. So I tested it, and sure enough it couldn't read the pin. I recompiled the code for a different pin, and retested it and it worked. Gave it to him, and it came back dead again. He wasn't connecting a ground between them, and luckily it was only taking out the single pin and not the whole chip.