There are two common causes of this problem...
1. FL Studio can't find the sample data associated with the Instrument Channel/s
Channels with missing sample data are colored red to warn you of the above. This can happen because the project file has attempted to load samples that -
Make sure that the sample's location is listed in the Search paths in the File settings window. FL will search its own data folder, as well as any paths listed as in the Search paths (and sub-directories of those folders) for samples. See also the Manual section Missing Plugins or Files for some more detailed strategies for locating moved/missing samples.
We occasionally drop samples from newer versions of FL Studio as we update the library to newer sounds, here they are: