shooting low

Bought a Ruger Air Hawk .177 break brl . It was shooting low with iron sights. Cranked up rear sight to zero and it was as high as it would go. looked silly being that high. windage was 2 marks over from center. It was like the front sight was too high. I wanted to keep the open sights, but did not like having the rear sight cranked up that high.
finally just put the scope on and sighted in. it groups ok now, haven't shot it much yet. Any one know why it would be shooting so low. I know you can't anailze without looking at the rifle I guess I'm asking is that out of the ordinary. I'm new at air guns, but I thought they were very accurate. Saw a video about the artillery hold have not had a chance to try it yet.