sometimes I see double images through my binoculars other times, not. What am I doing wrong?

This has nothing to do with setting the correct separation of the eyepieces for your eyes. The two lens barrels have become misaligned to each other. This causes a double image.

In your case the misalignment is small because you see a double image only part of the time. Fresh eyes can usually fuse two separated images. However, as your eyes get tired the images will separate.

I would advise not using these binoculars because the effort of fusing the images is likely to result in a headache. Buy a new pair.

Images Of Binoculars

The distance between your eyes is called your interpupillary distance (IPD). This number is measured in millimeters and is easily measured. Your eye doctor and opticians do it all the time. Good binos are adjustable and you can usually find the IPD that they will accommodate listed on the manufacturers website. A typical pair of binos MIGHT be adjustable from 55-72mm (I really don't know the ACTUAL avg available) which works well for 99% of people, but if a small child or a big guy (like myself) uses these binos the image may be distorted because their IPD is outside the size adjustment.

Binos can also become mis-collimated espically if they are dropped, but then the distortion would be seen all the time not intermittantly as you are experiencing.

Sounds like your binoculars are not well collimated. That means the left and right sides don’t align properly.

If they have a hinge in the middle there may be a bend or damage that causes them not to align except when spread to a certain width.

The two lens barrels are not in alignment with your eyes, causing an optical illusion. Your binoculars should have a hinge in the middle that allows you to adjust to your eye gap.

If they are fixed barrels, you can not adjust them, like a "one size fits all" baseball cap. Like heads, eye distance is different from person to person.

If they don't adjust, you may want to invest in another pair.

Not ajusting the distance between the eye pieces.

It's arguable and there are in fact more answers to this question...

I would change to a better brand of beer.