Never seem to get a positive OPK?

Finding it odd is anyone in the same boat? I begin testing the last day of my period and once I hit day 9 or 10 I'm testing 3x a day so i don't miss it on 2 different brands until cycle day 25 or 26 I've done this for 3 months now and never even get close to a positive. Is this something I should be worried about or is this normal for some people?