A little mess in Vocabulary practice
Normally, I don't wait until whole skills start to decay, so I use Vocabulary to check if there are less-than-perfect words for me. When I do "Lesson practice", I expect my weakest words to be strengthened, but it is not always so.
I noticed that Duolingo sometimes, indeed, gives me the words that are listed as weakest (if I sort Vocabulary by Strength in ascending order), but sometimes it gives me the less recently practiced words, even though they are shown with 4 golden bars. So, sometimes I do a few practice sessions in a row and the number of my less-than-4-bars words does not decrease.
I don't mind practicing words I have not seen for a few weeks, but I don't like this inconsistency. If the algorithm thinks it is time for me to practice these words, why does not it show them with 3 or 2 bars instead of 4?
This is just a guess, but maybe this has to do with something more than just the gold bars. For example, your error rate for those words or how many times you have seen them may also be factored into the Duolingo's decision of which words you should practice (the words could be new to you, so you still need to practice them (most likely); but they might have four gold bars because you have recently seen them, and they haven't decayed yet). Although, I must say, it does make sense that words with less gold bars would need to be practiced first, so you do have a good point.
The gold bars are supposed to be the resulting strength that takes into account everything: how long ago I saw the word, how many errors I made, etc.
but they might have four gold bars because you have recently seen them, and they haven't decayed yet
I wrote about less recently practiced words with 4 bars, so this is not the case.