Is there something wrong with the fitting or is this just a loose tray?

A few of my trays have been loose on my bottom left molars, only the ones in the very back, whenever I first put them in. I'm used to those because they end up getting tighter later in the week. But this week's and last week's trays were loose enough on my left side that I was able to take them off with my tongue. I just put on this week's trays an hour ago but last week's tray never tightened like others did. Is there something wrong with the fitting or is it just a casual loose tray?