I'm learning French and finding it working very well for the most part. Duolingo has been more helpful than any of the other several, previous methods I have tried.
However, some lessons are too difficult. I always do the strengthening exercises before going on to new material. 95% of the lessons I can complete within 10 minutes. But occasionally, I have encountered a lesson that I simply cannot complete no matter how long I spend. This can be very frustrating. It gives the impression that I have gone as far as possible with my abilities. But it's clearly because the lesson expects skills that have not been covered.
You should be able to monitor the courses for those exercises that are not completed more often than others and review and edit those to reduce their difficulty.
There's no doubt that some sections are more difficult than others and that some could use more explanatory notes. The most difficult exercise I ever did required 64 attempts to complete it. It may have been one of the subjunctive lessons. I can't remember. When things don't make sense I either search the Duolingo Discussions where often others will provide links that help explain matters, or do a Google search. Eventually, the extra time and effort helps my brain to see the light. Whatever you do, don't give up.
Never give up! Learning is frustrating, at least for most of us mere mortals! You may already know, but the brain actually learns more by making mistakes (it "auto-corrects" with trials!), so be easy on yourself! One tip that might help: I often open translate.google.com to use as a back-up when I'm not sure of vocabulary, but I am careful not to use it as a crutch or to "cheat" the system. DuoLingo is the best I've seen, short of personal interaction with others using the language (that's why we have friends!), although it's not perfect. Find what works best for you and cut yourself some slack; keep it fun!
Learning does not have to be frustrating. I just encountered another lesson impossible to complete. I went well beyond 70 tries with no progress. It is poor lesson design to have such uneven challenges and harmful to the learning process. translate.google.com was an interesting idea, and one I was not aware of, but did not help me complete the lesson at all. Duolingo would not accept the translations offered by google. Very frustrating!
Yes, 70 tries would be very frustrating, but it shows you have perseverance. Instead of using translate.google, try searching google by entering French Grammar plus your topic - e.g. French grammer - subjunctive or French grammar - adverbs. Or search such headings in Duo's Discussion section. You might find help there.