FTP checks every two weeks allow VeloPro to better tailor your plan to you. It's not a "test" per se, and so it's normal to not see any improvement (or little, or, depending on the previous days a bit of dip).
Regular checks are not to measure improvement, but to cue the system to keep you on track. Please note that we will soon release automatic FTP detection which will eliminate the need for FTP checks but in the meantime you can do the prescribed FTP checks...or not!
If you choose not to do the prescribed test, VeloPro will adjust. Conversely if you choose to do the test on a different date, you can mark it as such in VeloPro and the plan will adjust, if necessary.
On a related note, why are there workouts the day prior to an FTP test? FTP checks are meant to be a "real-world" measure of your current fitness and that's deliberate. If you only measure FTP when you are fresh, your numbers will be skewed too high, and you will feel as if you underperform in your subsequent workouts. So, FTP checks are done "in-line" with your workout schedule, while events have a taper before them. That said, you can always do an FTP check, or similar effort any time and mark it as such in VeloPro when you upload it.