I put a fresh reel of PLA in my printer and started printed really shiny things. ("Specular highlights" would be how 3D graphic artists would describe it.)
However, a few days later, the prints are coming out a bit matte.
I'm not terribly concerned, although it seems unusual to get such different results from the same reel.
What can possibly cause this?
In short, PLA filament is somewhat hydrophilic.
When you first unwrapped your reel of filament, it can be safely assumed that it came from the supplier nice and dry (unless you have a dodgy supplier). Hence your nice shiny prints.
After a few days, once unwrapped, that filament will have slowly absorbed humidity from the air in your room, unless you have a carefully controlled environment.
As the filament absorbs the water in the air, when going through the hot nozzle, that water will expand at a different rate to the filament, creating various gaps and inconsistencies in the resulting print. These inconsistencies will make the final print surface appear less shiny, or matte.
At the extreme, after an extended period of absorbing water, very damp filament will start to pop and crackle at the nozzle (as the water turns to steam) and create brittle prints with a very bad finish, with pits and bubbles and look very ugly.
I suggest sorting out the environment of the room, and at the very least get a dehumidifier or store the spool in a box filled with silica gel. There are other measures that can be taken, but that is beyond the scope of your question, which was about the matt finish.
See also Filament storage?
