Tried searching the site but didn't find any direct links. If this has been beat like a dead horse, please point me to thread. We are newish still wearing some green. Started in October 2020 with my daughter getting a Gskyer 70400. We had a blast with it. We bought a full line of Celestron Omni 40, 32, 15, 12, 9, 6, 4. All have been good but the 32 and 40.
Question is am I doing something wrong or are these just hard to use? There is only one very specific place you see the field otherwise the sides blur bad and almost cover the whole FOV. If you are patient enough the views are nice but it's hard to keep eye in one exact spot. We have used them in 70400 refractor Gskyer, a 70700 refractor Celestron power seeker, a 127x700 Celestron power seeker reflector, and a Celestron Omni 150 reflector.
Also if the eyepiece is the problem is the Tele Vue 32mm plossl a good one that won't do this. This price is $150 is top of my budget for a fix.
All good comments above. I'm guessing that it's simply the longer eye relief that's giving you a challenge. I use a 40mm Omni Plossl and 32mm TeleVue and Denkmeier Plossls. The typical problem with these is getting too close or weaving from side to side which moves your eye out of the proper position.
The eye relief of a Plossl is about 2/3rds the focal length of the eyepiece. So the 15mm Omni you use has about a 10mm eye relief, which means your eye is close to the eyepiece, and that's helping you with positioning. Your eye is even closer using the 12 through 4mm.
On the other hand, the 32mm and 40mm Plossls have 22mm and 27mm eye relief and your eye will not be close to the eyepiece, so you have no physical guide for positioning. Buying a different 32mm Plossl will probably not help, regardless of the brand unless it has some sort of extension that brings a physical eyecup/guard up to your eye. That's what the TeleVue Extender does for the TV32 and 40mm Plossls. But that's expensive and you can get the same result with your Omni with some creativity.
Some people wrap the eyepiece in a piece of black felt or some other soft material to raise the eyecup enough to contact your face. This can really help teach / maintain good eye position. Don at EyePieces, Etc - Starman1 on CN - wrote a procedure to use a piece of a bicycle innertube to make a soft extending eyecup. Search for that or contact Don.
My guess is that you'll earn to use the long eye relief Plossls with a little practice and end up liking them very much. Also, as said above, good body position helps keep your head still so the observing chair mentioned above by Jon is a good accessory.