If the same remotes are programmed to both units then you might have one with a failing battery or a failing remote altogether.
Otherwise the issue is probably some form of interference that began around the time your problems did and is limiting your range. Even though the units use advanced id coding, all signals are still sent in the same radio range, just like a radio station can play many different songs but they are all heard over the same radio frequency.When inside the garage or with the door open there is less interference with the signal, just like using a larger antenna or moving your radio to higher ground, so the units still work better.The troubleshooting advice in your Owner's Manual for short range issues, and the additional information in the links below should help further. Problem Solved..... It was interference and the source was newly installed Verizon FIOS (fiber optic) equipment cabinets installed on the garage wall together with a backup battery unit. The problem was solved by Verizon personnel changing out the FIOS unit for a different model.