Do you know how it is now fastened on - with nails, screws, glue, what? If it is any of those, it probably just needs retightening, if not, you need to find someone who can tell you about it by looking, unless you can afford to just get rid of it and get a new one put on, but be careful who you choose to do it as it needs to fit precisely, especially in winter.

Thanks for your reply. My husband stripped all of the old caulk off and it appears that is all that is holding it in across the threshold. Is that possible? We dont see any screws or nails along the bottom. Any suggestions?

joed might be right, but I have to ask first: is the hinge side of the door against a sidelight (window)? If it is, you need to be careful.

If it isn't, and you want to try something less obtrusive than pulling off your trim, replace one screw per hinge with a 3" long screw. (Preferably the hole that's closest to the center of the wall.) Do the same with the lock-side strike. You may still have to pull your trim and shim (as above), but there's a good chance this might work.

It is entirely possible, practically guaranteed as a matter of fact, that the threshold is not screwed or nailed. Normally the installer will put a couple of large beads of calk across the bottom and set the threshold in that and screw/nail the sides. also nothing in the top except trim nails.

The threshold is normally only connected to the frame. If the frame is moving then of course the threshold will be moving.

If you don't use shims you could distort the frame by installing screws.

Thanks to you all for your suggestions.

And yes, justnigel, the hinge side of the door is against a sidelight. It seems the entire thing (door and sidelights on either side) is all one piece...so I am thinking longer screws might not work. It seems the outer casing (not sure of the correct term) of the sidelight is what needs to be anchored better to the wall...but there is no where to anchor it without removing trim, etc...

Any suggestions??

Thanks again for all of the help!

Remove the trim. Investigate and re-anchor the door/sidelight unit. It sounds like what ever anchors were in there before have rusted or broken or were never installed properly.
Removing the trim is not that difficult. If it was painted cut the seams with sharp knife first. Use a putty knife and a flat bar to get behind it and pry it very carefully off. You only need the side trims off not the top.

Hmmm, I could've sworn that I already acknowledged joed's correction, but either the computer ate it or I dreamed it.

Anyway, joed noted that screwing through hinges (or anywhere in the door/window frame, for that matter) that don't have shims behind them could distort the frame, and he's absolutely right. In a perfect world, there are shims behind each hinge, but there's no guarantee.

In this particular case, if the join between the door and the sidelight is loose, you want to keep screws short enough to miss your sidelight in case you hit the glass or puncture the sealed unit.