I have just completed a strengthening exercise on Abstract Objects starting at a strength of 4/5, got all the answers correct (not a common event) and yet I am still shown as 4/5. This has happened before on this course, but not on any of the others I do. The "contact" button wouldn't work when I tried to report this as well. Is this a glitch on the English from Italian course only, and could someone at Duo please correct it as it's demoralising. Thanks.
There is no guarantee that one strengthening session will always complete one segment. The 4/5 indicator is based on the strength of all individual words in the skill but strengthening will not necessarily strengthen all weakest words.
In the "big" skills this is quite common. The strength is related to the percentage of "old" words in a skill, so for skills with many words it can take more than 20 words to go up by one step.