From what I have read the Italians failed to make much progress in attacking Austria-Hungary during the first world war and were eventually pushed back. Why was this?
Not really, the Italian Front saw a general stalemate much like the Western Front. Italy suffered a dramatic defeat at Caporetto in 1917, but in spite of internal unrest was not coming apart along ethnic seams like the Austro-Hungarian empire, and a leadership change in both government and on the front was enough to organize a rally and eventual victory.
There is certainly more room for discussion, but I actually wrote about the rout at Caporetto as part of a wider thread on the Italian Front over here you might be interested in.