Some people I play with expressed frustration with repeating spells inconsistently failing to add on the Shadow of the Demon Lord sheet, so I investigated a bit and it turns out it's a much more general problem. If you still have a previous repeating section that you've entered new values into and it's still in focus, adding a new section will fail. If you take focus off the previous repeating section before hitting "add", it's fine. If you don't change the values, it's fine. Reproduction: Write something in one repeating section, and attempt to add another section while the previous one is in focus. I've successfully reproduced the issue on the SotDL sheet, the roll20 5e OGL sheet, and the Dresden Files sheet (as shown in the illustrative gif below, since it's a very simple one to use as an example). Speculation: Based on what was said in this thread about disappearing repeating fields , I think I know what's going on. The previous field losing focus causes it to update, which happens on the server, so it occurs with a little bit of delay. The delay puts the update after the (client side) creation of the placeholder, and the update causes the placeholder to vanish because the placeholder has no data in it, and thus doesn't exist as far as the server is concerned. I honestly don't think this causes any usability issues on the 5e OGL PC sheet, specifically because the layout of that one strongly encourages the user to collapse the previous field before adding a new one (because the open spell/attack/whatever editing section takes up a lot of space). But it can be pretty awful to hit buttons and have them not work on sheets that don't guide user behavior in the same way, like the 5e OGL NPC sheet's initial repeating sections, the DFRPG sheet, or the one that caused me to look into this in the first place, the SotDL sheet.
