You could be having a LH surge and not actually releasing an egg. It happend to me for a few months till we figured out that was the issue!
in my case i too had used OPK which showed positive then negative after one day suggesting ovulation...but on follicle monitoring by TVS i found out that i was having anovulation that cycle....my doc said that though OPKs can give positive results they show only the LH surge and not ovulation....so my egg was not released ....i guess follicular study is more helpful than OPKs
On some OPKs, you will have two lines show up but it is not a positive OPK unless the test line is equally dark or darker than the control line. Try and reread the directions very carefully on what constitutes as a pos result. i misread an OPK because of this
if i understand correctly ovulating is the release of a mature follicle or egg from the ovary so I could be wrong but I don't think you can ovulate without the release of an egg but I can not explain OPK result either good luck and prayers be with you