The first evaluations of the B17 by the RAF were a failure. They had the B17C model which they called the Fortress Mk1. They attempted bombing from 30,000' and had very little sucess - no hits were recorded at all in the first two raids. And the guns froze. I suppose it found it hard to shake off this initial impression.
And subsequent marks failed to address the problems of the B17s limited load after more crew, defensive guns and armour were added. A typical B17 load was 4,500lbs of bombs. The Lancaster was carrying between 12 and 16,000lbs, and modified eventually carried 22,000lbs. Even the Mosquito could manage 4,000lbs.