right click the vi icon (top right)
Vi properties....>windows appearance--->
Custom--->customize
make sure that "show front panel when called" or "show front panel when loaded" are not checked.
also, in the main vi where the sub vi's are called,
right click on the subvi, ---->subvi node setup, make sure the options are not selectedb either
also examine the code for any invoke node that opens the subvi programmatically