Whenever I open my shared excel file, the changes i had earlier made are all found undone

I create an excel file and shared it to two other users through Google drive. The other users have 'edit authority' to my file and they do so by using Google Sheets App in their smart phones. One night I just modified my file did some formatting, carefully saved and closed the file. Next day when I opened the file it was like no changes were made ever. All the changes that I made last night weren't there. I again perform all the work done before, saved and closed but the next day found the same problem. When I checked for edit history it says it was opened by one user but he neither made any changes nor could he see my changes. Is there something with Synchronization? I don't know what's that. Please help. Your concern is highly appreciated and regarded.

Thank You.