Reusing clear blue advance digital opk

Leonie
Last month I only used 3 sticks out of the pack before I reached my peak. This month I have carried on using the same holder and the remaining sticks to keep getting a low result even though I am due to ovulate the days I'm testing. 
Could this be because you can not reuse the reader once you've hit peak or could it be I'm just not ovulating (although I have cramps as though I am)?