D&D 5e monsters no longer come with character sheet

I've been DMing online for a couple of months now, and up until recently I could drag and drop any monster for the monster manual and it came with a clickable sheet that I could use to roll attacks, etc. Now, however, unless it's a monster that I've previously used, all i get is the monster's page from the Monster Manual (which I did not used to get) with no tab for the character sheet. What is going on?

It sounds like you created a game but forgot to choose a character sheet? If you are still seeing a character sheet tab elsewhere in the same game, it sounds like perhaps you are dragging handouts instead of NPCs?

I have character sheets, and I have been playing this game for months and only recently been unable to get monsters' character sheets. Also it only works with some monsters. As an example, I added a Gynosphinx from the compendium and then the gynosphinx showed up as a character sheet in the journal and I can click on attacks, etc. However, when I added an Umber Hulk, the token comes with HP and AC and everything, but it doesn't show up in the journal and when I click on it in the compendium all I get is the Monster Manual page.

So which character sheet are you using? If you are not using the D&D 5e by Roll20 sheet you may get inconsistent results. Also, a screenshot of a problematic character's window and tabs might help.

What happens with the Umber Hulk if you have the token selected and Shift + double click or Alt + double click?  If that brings up a sheet, then it is in your journal... somewhere.

Shift + Double Click worked! It still doesn't show up in the journal, but it does give me clickable character sheets, so it's good enough for me. Thank you!

The sheet has to be there somewhere, or it wouldn't come up when you do that.  If you are absolutely sure it isn't in any of your folders in your journal, you might check to see if there are any archived character sheets in your game.  It might be archived.