This has been a recurring issue that happens to people since 3.4.x came out.
Some people get this issue, while others do not.
Some people fix it by removing the user profile [in various places depending on the system], and then allowing LO to recreate it.
Others fixed the problem with a reinstall of the package after they removed the user profile.
Still others fixed it by downloading/installing a dictionary extension file [.oxt] for the language you want to use. I try to maintain an up-to-date list of all the available dictionaries that I know of.
http://libreoffice-na.us/English-3.5-installs/dictionary.html
with over 180 dictionaries, including over 20 localized Spanish dictionaries.
The official LO extension center also has dictionary files.
My largest American English dictionary/thesaurus has over 773,000 words in its spell checking word list. I also have British and Canadian English dictionaries with over 600,000 words in it.
I would rename the user profile [Ubuntu Linux has it in a hidden .libreoffice folder, but I do not remember where the different Windows ones are located]. Then I would reinstall the LO version you are using. Then to give you some extra "help", I would download an localized dictionaries you might want to use and install them with the Extension Manager. That will cover all your bases and may give you some more localized dictionary options that the "normal" LO install does not cover.
The reasons why the Spell-checking issue happens to some but not others, is a mystery to many of us. But these steps usually correct the problem.