You've probably done these, but just in case:
Blood ON the strip, or IN the strip? It is supposed to "sip" blood into the little channel opening at the end, not wipe a drop on top of the flat surface.
Check the date on the container - they can get out of date.
Make sure you keep it tightly capped, in a not hot, nor damp location. If the storage location was hot or damp, and the cap wasn't tight, it can affect the strips.
Make sure your fingers are clean and dry before handling. It bugs me that they are produced with the test end towards the opening to pull them out, because it makes it easy to contaminate them with water, etc.
Check that the strip was fully inserted - it gives an initial beep. You must insert it before applying the blood.