Check your lands - I played multiple games of a three-colour sealed deck with 12 lands, 10 of them basic swamps
My first sealed deck of the format - opened it a few days ago and put together what looked like a okay-ish Grixis deck, but didn't have time to play any matches. Came back to it this morning and fired it straight up, only to get mana screwed game one; lots of red cards stuck in my hand, one green-black duel, and otherwise nothing but swamps.
Annoying, but it happens. I suck it up and move on to game two, opening what looks like a very respectable hand, though it only has green (via a green-black land) and black lands. Yet again I get mana screwed and even though my opponent also stumbles they recover before I do and eventually I run out of black removal and lose my second game.
At this point I pull up the actual deck... to find I'm somehow running ten swamps, a green-black land and a red-black land... and that's it. I guess I never did finish deciding on lands when I was deck building a few days ago. Oops.
On the plus side, I get to cut some of the more mediocre playables now...