One of the most important factors in using eye drops for glaucoma is regularity over a long period of time. Forgetting to put your eyedrops may happen once in a while, but consistent/repetitive failure to comply with eyedrop treatment may result in poor glaucoma control and vision loss.
If you forget to take a dose of your eye drops, it is best to instill the drop as soon as you remember it. If the next dose is due shortly, wait for the usual time to use it.