How long have you had it?
It might be normal if your jardini has always done that.
If it is a change in behaviour then maybe somethings wrong.
I know my Jar is very active.. always swimming near the top.
if the jar is new then it might be normal for it to do that. when i bought my new jar it stayed at the bottom too for a couple of days. when it got adjusted to the tank it starting swimming freely.