You do have ALL of the monsters you used as that Synchro's material in your Graveyard, right? And make sure that you have enough zones since you have to Special Summon all of them.
Yup. Check, check, and check. All in the GY, enough monster zones, and no special summon limitations. The only thing I can think of is I used a spell to change one of there levels for the synchro summon, but that shouldn't matter right?
literally just happened to me, and i was angry as hell, lol. came to the internet to see if anyone experienced this "fluke", only to find out it wasn't a fluke and i was just careless, lmao. thanks for explaining, kyoko.