ok, i guess you are using this theme ?!:
https://www.rockettheme.com/wordpress-themes/momentum
as its not free – and i’m not really prepared to spend 50$ just to test one thing for one minute – we can/should do 2 things
a) in your install just briefly revert to the standard twenty12 (or twenty11 or whataver it is) and see if it still does or does not display.
if it DOES display, its the theme that causes the problem (which is my guess anyway, but who knows)
b)if we find out that it is the theme that causes the problem, maybe you can give me a url i can have a look at (where the widget is installed)
my guess is as follows though: when inserting a widget somewhere, your theme does not include the actual id into the wiget element. although the plugin contains code to deal with most eventualities to insert those id’s regardless – even when the theme does not insert the id – there may still be situations where it can’t sort out/insert those missing id’s
anyway, one step at a time.
if i can have a look at your install somehow to start off with, we can probably find out what’s going on…