duolingos helpful - most people usually get over it, but if you stick it out and push right to the end there is some seriously awesome vocabulary and levels at the end
also if youve gotten to a late-game stage where you can do everything grammar wise you just need to learn more words you can try one of the following things
- if there is a book that you know really well and have read heaps of times, read that book in the language youre learning (this one definitely works if you can persist, though i dont think it has the best value for time)
- you could aim to learn 1 or 2 new words a day, write it out in your new language and english like 20 times and boom you know it, revise weekly and monthly and then yearly, youll be happy with your progress !!
- go overseas and immerse in the culture, this one is arguably the most effective and quickest for people at the plateau stage though it is expensive - but usually a good experience
best of luck !!