You need to activate all 4 puzzles.
To unlock all 26 alphabetic forms, all four slide puzzles must be activated. -Bulbepedia
That's for the 26 letters. Now for ! and ?
Once the first seven pages of the report are complete and the 26 alphabetic Unown forms have been caught, the entrance room of the ruins will have a gold plate that reads "Our friends look up and heartily congratulate us". Reading the plate causes one of the researchers to speak to the player, after which the ! and ? forms can be encountered in the entrance room. -Bulbepedia
Hope This Helped :D