I'd actually start by getting a BBT and start charting....it'll help you pinpoint when you ovulate!
You can order the BBT online, or some stores carry them [as long as it reads out to the 100th place, it'll work great [98.76]]
There's a free charting site: wwwfertilityfriend.com, there are a couple videos that will teach you the ins and outs of charting, plus you get free VIP for a month so they will figure out your cycle for yoU!
There is no instant gratification with OPKs of BBTs. With OPKs, you ovulate 12-24 hours after a positive and with BBT, you will know you ovulated by having 3 higher temperatures [96.00, 97.03, 97.86, 98.02] You'll notice an obvious temperature rise.
Either way, the videos on Fertilityfriend.com are GREAT! I learned so much from them!
With the OPKs, when did you start using them? Your luteal phase [after ovulation/2ww] should be between 12-14 days long] so if you're having a 25 day cycle, you are most likely ovulating between CD/9/CD10/CD11....so if you don't start OPKing until CD12/13 you may just be missing your LH surge [the hormone the OPK detects]
Good luck! Hopefully I answered everything. lol