I find I have to go to Options -> Corrections and uncheck and then re-check ‘Check spelling as you type’, and then press the Apply (or OK) button, to get Scrivener on Linux (Ubuntu 12.04 LTS, 64-bit, Unity UI) to actually underline in red with squiggly lines my spelling errors (see image, below). I don’t always remember to do so, and thus sometimes send along document drafts to colleagues with spelling error in them that I should have noticed and corrected.
I can’t produce this bug (Ubuntu 15.04 64-bit, Scrivener 1.7.2.3).
I wonder if it’s an issue with 12.04. I was running Scrivener 1.7.2.3 on Ubuntu 14.04 for a whle and spellcheck was fine, so I suppose you could try upgrading Ubuntu to the newer LTS.
Do you have the latest version of libaspell-dev installed (and the aspell dictionary for your preferred language)?