My friend's 2007 Trek 2100 WSD is having trouble shifting to the smallest chainring. It's a 105 triple with an Ultegra front derailer and shifts seamlessly from the middle to big chainrings. The derailer moves to try and shift the chain to the front, but it never wants to shift down.

I tried adjusting one of the limit screws and saw the derailer moving in and out. I tried moving it in until turning the limit screw would move it no more, but it still won't shift into the small chainring.

Any other suggestions?

EDIT: Argh. Just realized that I posted this in 41 instead of in the Bike Mechanics forum...

has the deraiilleur ever been cleaned?
has the cable been cleaned or replaced?
a 2007 will do well with a new cable (or good cleaning by someone who knows...) especially with lower mileage.
is the underside of the BB shell clean? a lot of gunk collects down there and restricts good cable movement.
when the shift is down to the granny (without moving the FD by hand...) - check the cable tension about mid downtube, if it's loose and easily moved, something is keeping the cable/FD from completing full movement.
it's common for cables to get rusty/cruddy in that section which has to slide under the BB when shifting to the granny, because most riders use the granny very little. More shifting from Outer ring to middle (and back) keeps that cable section a little more operationally active and 'clean'.
if someone has fooled with the FD, check for proper clamp height and proper cage alignment.
105 Triple Cranks and Triple FDs? bulletproof
but they work best when the shift system is clean.
and now having screwed with the FD adjustments, you;d do well to get it properly adjusted as soon as the hangups are handled.