My experience as well. Recently flew LIM-PTY-LAX on 016/UA ticket on CM/Copa metal. LIM-PTY delayed enough that I missed PTY-LAX. Called UA to rebook on next CM PTY-LAX flight; UA rebooked it. Could not check in on Copa app. Called Copa; Copa explained that there was some problem on the UA end that Copa could not solve. Called UA and was told problem was solved. Again tried to check in on Copa app, but could not. Again called Copa; Copa explained that there was some problem on the UA end that Copa could not solve. Again called UA and was problem finally was solved.