As a trouble shooting step, make sure that a commit records is being performed to actually "save" the new data back to the database. This can be done via a script or by tapping a blank area of the layout.
Thank you for your post.
Does this issue occur after long period of use? If you relaunch FileMaker Go, open the file, are you then able to save the data in the related table?
Is it possible you have lost connection with the host?
Does this issue also occur with local files?
Any other information you can provide may be helpful in narrowing down the possible causes.