It is normal for a non-fixed bottom orthodontic retainer to be slightly loose?

Slightly, Yes: You should have your dentist evaluate why it is loose. If it is not adequately holding the teeth where they were moved by orthodontics, then you may need a new retainer, a fixed retainer or a slight tweek to get the teeth back in place. Do not wait to long, as you have probably spent a lot of time and money on the orthodontics and want to avoid significant relapse.

Othodontics: There are different types of appliances that apply pressures on teeth in various positions. With time they can get loose as the teeth are brought into the proper position. It is a gradual process . See your orthodontist that fabricated the appliance to have it checked.

Yes: A removable retainer will become slightly loose over time. Just be sure to examine it for cracking which can affect fit. As long as it is intact, the fact that it is loosening indicates that you are wearing it adequately. An orthodontist becomes concerned when a retainer tightens over time, because it indicates that teeth are moving and/or the retainer is not being worn enough.

Yes: In my office we say "loose is good, tight is bad". A loose fitting retainer means that the teeth are staying in position, a tight retainer indicates your teeth are moving and you need to wear your retainer more. If it is a plastic and wire retainer , it is possible that the wire have relaxed some and may require slight adjustment.

No: They should fit snug. See your orthodontists soon.

Loose retainer: Yes, with good regular wear, it is common for a retainer to become "slightly loose." this is one of the reasons regular visits to your orthodontist, over many years, even infrequently, can be very advantageous. Your retainer may be adjusted.

Yes: Well fitting retainers loosen with time. See your orthodontist for what is probably a very minor adjustment.