There could be a number of things that may effect your DRS.
- You are heavier, the car is carrying more fuel
- Your tire temperatures maybe too high or too low
- You may have component wear that is reducing your speed
- You could simply be far too far back, 0.999 seconds will never get you an overtake without a drs assist.
- Your driver's health maybe too low, and he is losing speed cause of it.
- Your tires maybe losing their grip and you lost traction.
Whatever the issue is, DRS is a little bit slow but not slow enough to be boosted.