When to create distance

I sparred with my coach yesterday, he said I did a great job at shelling up and throwing the counter (especially on the left side), but he said he’d like to see me create more distance when someone is throwing on me as opposed to just shelling up and blocking punches or countering. It gets to a point that where I’m shelling up guys are just laying it on me and I’ll just move my head and slip or throw back on the counter or just stay shelled up. I think its important to note that my defense is good enough that I’m not taking shots, but guys are just unloading on me. When is it appropriate to create that distance?