Hmm...just looked up the 3GR instructions to check where the button is...would appear that there isn't a sync button at all...so given you duff info by the looks of it
Instructions just say put the tx within 1m of rx, turn on tx, make sure that led on the back of tx is solid green, then turn on rx and then press the 'sw' button, when the rx led turns solid green it's linked.
If rx led is blinking green then it's receiving signal but unmatched...solid green when matched...red when no signal being received at all.
So if you havent already, try holding the button down til it's solid maybe? Really not sure, just did it straight off when I did it last
No other idea's I'm affraid?!?!