I have retrieved the Staff of Magnus and returned to Winterhold, and spoken to Tolfdir about what to do next.

I'm supposed to use the Staff of Magnus to breach the barrier preventing entry to the college. Unfortunately, when I use the staff, nothing happens. It marks up the ramp beyond the barrier, but the barrier doesn't change at all. I have emptied the staff's charge into the barrier with no result.

I am rather stuck here, since I can't get inside to fight Ancano.

I have the exact same problem, tried everything I could find/think of, and nothing.

Go back and talk to Tolfdir again; sometimes the quest doesn't trigger. This just happened to me last night. I had to walk around to just the right spot for it to work.

I'm having this same issue, tried talking to him several times...tried reloading...nothing is working.

Had a similar issue in which Tolfdir would spawn inside the barrier while fighting the magic anomalies so I couldn't talk to him. However using the staff on the barrier for a couple of seconds worked regardless and I could enter without even talking to Tolfdir.

I never used the staff on the barrier, if i remember correctly. I used frost/fire on it tho with Tolfdir helping, inside it felt at times it wasnt working.