These fall in to two broad categories – Those that aren’t available, and those that won’t work well
Although most ZX Spectrum games are freely available, the copyright holders of some games (notably Ultimate Play The Game) have not allowed their games to be distributed. Please check on World of Spectrum to see if the game you are looking for is available for download. If it is not, then, sorry, but I am unable to supply it. Even if you are able to find a copy elsewhere, if WoS lists distribution as denied then I won’t make a copy of it.
Some games, whilst being available, don’t work too well in this medium. Games with multiple parts to load in, or that rely on tapes being at certain positions don’t translate well to the ZX SpectROM. Deus Ex Machina, for example, has narration on the tape that is needed to get the most out of the game. Whilst these kind of games were quite novel, your enjoyment on SpectROM might be diminished.