LyX ⇒ Updated lyx-file fails to be updated in pdf-export

[Mr_Stone]

Hello,

I do not know what is wrong with lyx. I am working on a lyx document, that has 3 subdocuments. I am updating the text in one of these subdocuments, but when i want to see the updated pdf, a new pdf is generated, but it does not contain an updated version of my document. The pdf contains the same text it had when i began the lyx-session.
If I use another export-type it updates correctly but i have to switch after each export...

UPDATE: If i change something in my "main" document the update works correctly
I am using WIN7 and lyx 2.1.1