Tomo Takeuchi's declining health forced her to halt work on her hit manga series Welcome to the Ballroom, just as it gaining popularity in North America thanks to a 24-episode TV anime adaptation from Production I.G. The hiatus began in December 2017 and was initially stated to last for only 2 months but to this day, the series hasn't returned to the pages of Monthly Shonen Magazine.
Currently, there are 9 collected volumes of the series, with the anime having covered all 9 volumes. Without fresh manga chapaters, the anime obviously couldn't continue.
Well, thanks to Takeuchi's recent Twitter activity, it seems a return is on the horizon.