Are you switching out your N95 throughout the day? If the exposure is high enough you can overload the mask and have break through.
Also, have you had the mask fit-tested? It's possible that the mask has some gaps that could let some of the fume get sucked in. There is an entire procedure to fit-test your mask but if you want to do a quick check try the following: Place you hands over the mask and breath out hard and quick. Try to feel if any air escapes under your mask/face seal. If you feel a leak you might want to try another brand.
One last thing, make sure that you shave before using an N95 and try not to have anything under the mask's elastic bands (like glasses, a ponytail, baseball cap)
Hope that helps
-David
