See this recent thread ...
https://www.mobileread.com/forums/sh...d.php?t=337751
which has many things to check and a nice summary by Tex2000ans about spellchecking.
So what platform?
What version of Sigil?
What Primary and Secondary Dictionaries have you selected in Sigil Preferences?
What have you set for dc:language in the metadata of the OPF?
What if any xml:lang and lang attributes are you using in your xhtml code?
Which dictionaries have you actually installed and how are they named?
If you built Sigil itself on Linux make sure you have followed the docs closely and know where system hunspell dictionaries are located?
All of this may matter.