Specialized makes a rubber shim for Shimano brifters. I have used them on both Ultegra and Dura Ace brifters and both work very well making a significant difference for my small hands. I had to order the Dura Ace shims direct from Specialized but most Specialized dealers tend to have the 105/Ultegra shims on hand. This may be a quick and inexpensive way to fix the problem.