How do you get suction cups to stay on a window?

Your windows must be scrupulously clean, and free of any condensation. Any possibility of humidity changes may affect the suction to the window. If decorations are fragile, it may be wiser to suspend them from a rod, chandelier, etc., where they can be secured and won't be disturbed.

You could use clear fishing line and hang them from a tack put in at the top of the window frame. Did you wet the suction cup? Sometimes that will get them to hold better. Olivia is right, though, the glass must be scrupulously clean for them to hold.

I find that cleaning the window and then getting the suction cup slightly moist will help it stick. If the decorations are fragile, expensive, or important, I'd find a different way to hang them. Could you use fishing wire to hang them from the top of the window?

Unfortunately the quality of these suction cups is inconsistent and can vary considerably. Also, they are designed, in theory, to work on windows that are perfectly dry. Any humidity will interfer with the grip.

How about using a tension rod inside the window casing instead?

I wouldn't trust hanging fragile decor on suction cups. Maybe Command brand hooks?

Using Command is a good suggestion. I think the problem is that there is a thin film of grease or oil on the window. I usually use straight bleach but any dish cleaner that removes grease should work. I hope you get those things to stay up! Another idea I have done is to tack something that you can tie a string to on the top part of a window -in outside or outside of the frame , Hang your decorative things from string ties with strings which you have tied to the top. Either straight across or sometimes I like to hang things in a pattern. Like different length of string

Good luck !