I have a 2011 Tarmac SL3 pro with the red group, I replaced the stock cassette with a SRAM red 11-23 cassette. I have roughly 1000 miles on the bike. On Sunday I rode a century and about halfway through I noticed that I could not shift to my lowest gear on the rear. This happens in either of my front chain rings. When I push the lever it will make the shift, but when I release pressure it returns to the next higher gear.

After the ride I put the bike on the stand and played with the low limit screw on the derailleur but there was no change. It seems that the problem has to be in the shifter. It is not holding that gear position. Has anyone here experienced anything like this?