I had a similar issue with a 1st Series .177 when using the leaf sights but the peep works fine. My other examples in all 3 calibres all shoot very well with opens and peeps. Ironically I have zeroed the peep sight on a '4th' Series to the .22 barrel and backsight with the .177 and enjoyed excellent results with both in competition.
Could it be the pellets you are using? I use FTT or Hobbies in the small bores and old 19 gr Milbro Rhino in the .25.
John