The wheels/runners/door Cams run in grooves on the top and bottom metal rails.

When installing them, the upper wheels should be fitted to the glass first, then hooked into place. Position the lower wheels in position through the glass and into the runners.

DO NOT PUSH THE WHEELS INTO THE RAIL - THIS WILL CRACK, WEAKEN AND DAMAGE THEM. DO NOT USE FORCE.

Once fitted regular maintenance should be undertaken to ensure smooth free gliding.
This should include:

Regularly check the doors meet in the middle 'true' if not, adjust the cams until they are correctly aligned. If not this will both allow water to escape and also put additional strain on the wheels

Soap scum and other bath build-ups will clog the bearings of the wheels leading to failure. Keep the runner tracks clear.

Using Silicone spray will help the wheels glide smoothly.