I am still getting used to SW and I am starting with easy things (thought it was an easy thing) like making a cylinder with threads. The trouble I am having is that the extrude won't take.

I start a new part

select the top plane and draw a circle.

extrude the circle

select the top face of the cylinder

draw a circle slightly smaill than the original sketch

extrude the new sketch

I click on the check mark and even though it shows in yellow that the extrusion looks good but when I click to complete the extrusion, it doesn't work.

I have to qualify the last sentense by saying "sometimes". What am I doing wrong?