I was under the same impression, but later the problem disappeared. You have to give Duolingo a couple of months of learning effort first, so that Duolingo has a larger base of sentences that you should already understand.
I used the app for 6 months (granted this was around 5 years ago) and the problem never disappeared for me.
To answer the question, it depends on which language you're learning. Japanese and Spanish probably have the most resources for English-speaking learners.
I was under the same impression, but later the problem disappeared. You have to give Duolingo a couple of months of learning effort first, so that Duolingo has a larger base of sentences that you should already understand.