Only molecules with a chromophore will show up under a UV lamp.
General rule: if a compound doesn't contain an aromatic ring (like benzene), it won't show up under a UV.

TLC spots are usually invisible to the naked eye, and not all molecules show up under UV. So how do you "spot" your spots then?
By using a developing agent, such as iodine or KMnO4.

If you put your TLC plate into a chamber filled with some iodine pellets, after a minute or so the spots will turn brown.

For TLC developing solutions like KMnO4 or "Hannesian dip" you use give your TLC a quick dip, and then heat it up under a heat gun, and the spots will change color.

Why do the spots change color? Because iodine, MnO4 react with just about every chemical, and cause a color change. Remember that the actual amount of chemical in a TLC spot is very small- something like one-millionth of a gram, so by comparison you're adding a huge amount of iodine (or KMnO4, etc.). So just about every spot will react and change color.