Hi, the other day I decided to clean up my board and converted a bunch of items into subitems in order for everything to be grouped. I had maybe 5 groups with as much as 25 sub items each. Everything looked amazing. A couple of hours went by, I opened my board again and the groups were still there but most of the sub items were gone. Some groups were empty, some others had 1 or 2 sub items left. After almost throwing my computer out the window I discovered the “undo” feature and managed to recover everything to its starting point (plain items). On Friday I decided to try again and grouped everything together. It’s Sunday night, I log in and… surprise… same thing happened all over again. Am I doing something wrong? Is it a buggy feature? Help please.
When you select all the items to convert them, the popup window should show you where the data is coming from, you can then choose column by column where the data from each line item is going on the new board. Often there is a column that has a different title, or something doesnt match exactly and that information will be lost unless the destination is chosen. Before you complete the conversion, the blue “approve change” button will say something like 20/22 (make sure it is 22/22, meaning no data is getting lost). good luck, sorry for no pictures.
Hey @pdearagon 
As @Amanda115 mentioned, you do need to make sure the same columns and labels are available in the subitems/items and then map the columns out accordingly when converting one to the other or else the data will be deleted as it has nowhere to go.
You can think of it as if you are moving an item to a new board when you turn a subitem into an item and vice versa 
You can also find the Full instructions here: Convert Subitem to item (and vice versa!)
I hope this helps!
Having the same issue. I have created duplicate columns in my subitems of the same type (text, numbers, etc.) When I convert to subitems the data disappears.
I am having this exact issue. I have manually recreated all columns, ensured that all data in dropdowns and status columns are identical in the subitems as they are in the original items and it seems to leave out moving any text in text field and status information does not move across either. This is massively frustrating!!!
I had this issue. When I converted not of the columns were showing! I converted the sub-items back items and all the data was there 
What I did was converted it back to a sub-item and refreshed the page - Viola!
Seems to simple, but it worked for me.