When am playing challenges after solving all questions to the last it delays a bit before concluding but of i mistakenly click the back arrow, ot returns back without prompting me , then if o return to the game its marks me wrong after passing the questions.
This is why i mostly loose challenges i.e the last question
Perhaps going from the first to the last question is quick because all the questions and answers have been downloaded into the App but at the end of the challenge, when the App needs to communicate with the server again to tally up your total xp etc. you experience connectivity issues due to SL server load.
May be Sololearn server is so busy
And you press the back arrow and lost the challenge so it is your fault .Not a fault of sololearn .
So many people are here on sololearn that why it is cause of network error