After releasing the first umbilical line (M4) with little trouble, Doug Wheelock moved onto unlatching the second (M3) 'quick disconnect' cable feeding ammonia into the failed pump module and ran into considerable difficulty.
Reporting seeing "snowflakes" -- bits of frozen ammonia -- drift free from the line, Wheelock was told by flight controllers to try cycling the quick-disconnect latch. Doing so, he found he was unable to open it again, even exerting appreciable force, due to what was assumed to be pressure in the line.
As the space station passed from day into night, Mission Control had hoped cooling the line would release some of the pressure, but to no avail. Ultimately, it was decided to move to the next line (M2) in the series, the idea being that by connecting it to a jumper box, it would help release the pressure from the M3 line.
A little over five hours into today's spacewalk, Wheelock and Tracy Caldwell Dyson are running significantly behind in their planned work due to the time they have spent troubleshooting the stubborn valve.