Raghu's advice is correct.
It is better to give credit for a problem rather than delete it once students start working on a homework set. Deleting a question from a homework set potentially changes all of the later question numbers which causes confusion in grading. (This can't be fixed without a major overhaul of the database design -- so for now-- once a homework set is being used don't delete any questions from it.)
WeBWorK Main Forum
Deleting a problem from active assignment
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day