Sounds like your chosen spellcheck dictionary doesn’t have the vocabulary you need – that’s not surprising if your “large words” are in a specialty jargon (medical, engineering, etc). You may need to add words to the custom dictionary.

Assuming you’ve chosen the Hunspell dictionary option in DSpellCheck, and assuming you’ve added at least one word to your custom dictionary: you can go to the DSpellCheck settings, grab the directory name from the Hunspell Dictionaries Path field, and then in that directory, open en_US.usr (or en_GB.usr, or en_AU.usr, or whichever flavor of English you’ve chosen for DSpellCheck). Then you can add all the technical/jargon/“large words” that are missing in that location. After you’ve saved the .usr file, restart Notepad++, and DSpellCheck will recognize all those words.