Jan Heine covers this in a blog article here https://janheine.wordpress.com/2013/03/25/trouble-with-sti-triples/ (and I imagine the wrong chainline might also exacerbate this for a given size of middle ring):
Many front derailleurs for triples have a channel pressed into the inner cage. This is designed to lift the chain onto the big chainring when you upshift. It works only if the channel matches the position of the chain on the middle ring. A derailleur like this works only with a very narrow range of chainring combinations.
If you use a differently-sized middle ring, the channel no longer lines up with the chain as you start the shift. In the photo above, the channel is above the chain. The chain gets stuck below the channel, and it’s almost impossible to shift to the large ring.
The article goes on to advise using a front derailleur without the indent in such circumstances, and suggests the Shimano CX70 model, which it says can work with triples.