I also have been having this problem.
I'm using Lingvist for three languages: Spanish, German, and Russian. Oddly, I thought I knew German best of all, Spanish next best, and Russian worst, but it said that I knew far more words in Spanish relative to German, which puzzles me, I feel like my reading and listening comprehension are superior in German to Spanish.
I notice a weird disparity too in the speed through which I'm advancing in German. For example, with today's stats, for Spanish, I got 83% correct repeats, and was given 42 new words. For German, I did 100 words and was not given a single new word!!! I went extra far to meet and exceed my new word goal...I did 159 words in the end...had 88% correct repeats, and was only given 29 new words.
I don't understand this. Even in Russian, where I perform poorly, I am given more new words. For example, today in Russian I did 106 cards, had a low correct repeat rate of only 59%, but was still given 20 new words.
This all makes no sense to me. I would expect to be given more new words if I'm getting more stuff correct.
It's also frustrating and unfortunate to hear that the customer experience here is poor. I have been using DuoLingo for the past two years and I have recently gotten really frustrated with it because the team there is so incredibly non-communicative and non-responsive, I've reported numerous bugs and seen probably 90%+ of the bugs I report go un-fixed for months, some of them years even...I have submitted dozens of bug reports but never received a personalized reply, only automated ones.
I would like to think that this site is better...it at least seems less buggy...but I really wish the team were responsive. Maybe I can try submitting a bug report or contacting them about this and seeing if they can address this.