Adjustments can slip back out of place, usually due to a muscle spasms pulling it out, or muscle memory if it was out for a long time...
The body may think the old position was the normal one, so tries to go back to that.
Then you need to get re adjusted or checked for that..it may take more than a few visit for it to stay in place.
If spasms /muscle tightness is the cause , those need to be addressed for best results.

The chiro I went to did soft tissue work as well as adjusting..