It is not unusual for riders who are steady climbers and endurance-centered in their abilities to suffer with AC efforts. Your training buddies probably usually beat you in the sprints!
In order to get the most from your 40/20 efforts (40 seconds on/20 seconds off), you'll want to start hard and finish all-out. So, instead of jumping right into the target wattage, start a bit lower and finish the 40 secs all-out.
What's important is the intervals themselves - they train you to go hard and recover faster and this, in turn, will help to train your AC abilities (at least to whatever you're naturally capable of).