All slimes can deflate over time and this is very natural and unavoidable since slime is mainly made out of glue and water. This process varies depending on type of slime (clay/butter/self-inflating slime tends to be deflated faster than other types of slimes) and use, as repeated exposure to air and skin will make your slime deflate.

But water&activator will fix that right up! Try to add small amount of water at a time until your desire texture and volume. If it feels sticky after adding water, that's completely normal, add tiny amounts of activator at a time.