Like the title suggests, my Fast Passes (all) keep disappearing before being able to be used. I will login, check how much a novel needs for a chapter and, for example will say I have 2, chapter requires 3. Go to earn fast passes by watching a video etc., go back and it now tells me I have 1 fast pass, require 3. 
I have also experienced checking, I have 1 fast pass, requires 2. Collect. Come back to be told 2 fast passes, require 3 for the same chapter.
Fast Passes expire after 7 days of obtaining them
Ella_Sheridan fastpass expire around 7 days after getting them
Ella_Sheridan
As others have noted, FPs expire after a set time.
However, there are also different types of FP.
You can only use FP that are compatible with the chapter you are trying to unlock.
Site-wide FP = Works on any chapter
Novel FP = Works on any novel chapter
Comic FP = works on any comic chapter
Genre FP = works on any chapter that matches the 'Genre Tag'. ie. 'Romance', 'Sci-fi', etc.
Specific-Works FP = will only work on the novel or comic that is named in it's description.
The problem is that most the time when you see a total of how many FP you have, it gives you a total of ALL FP, regardless of if you can use them or not.
The only exception is in the app at the point in a chapter when it prompts you to spend your FP to unlock that chapter. This shows the number of 'usable' FP for that chapter.
