I mistakenly created a collection when only one file was highlighted. I made another collection with all the other files I wanted in it, but I can't drag that one file from the filmstrip...
There must be another way!
I can drag it from the center, the destination collection highlights in blue and there is a plus sign, but the file does not enter the destination collection. I've tried some other collections, but the single file remains in its collection.
Sounds like that image is also in the second collection you made. Check and see. Lightroom will not move an image from one collection to another if it already resides in that collection. Which is a great feature. Stops you duplicating images inadvertantly.