I soaked chia seeds in milk overnight and kept them in the fridge. The next morning I did not find them very swelled.
What can I do to improve this.
It is possible that the seeds did not look like they had swelled. While the seed itself looks like its size stays about the same, the "swelling" really comes from the fiber soaking up liquid, forming a gelatinous "blob" around each seed pod.
And how much they swell will also depend on how much liquid they're soaking in. This is a time-lapse video of chia seeds swelling as they soak in water.
But always better safe than sorry: If you don't feel comfortable eating what you've made because you don't think it reacted properly, discard it and try again with a different batch. The more you experiment with chia seeds, the more you'll know what behavior to expect from them.