Infertility issues

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This is not a post about "How can I get pregnant faster?". 
In April 2015 I found out that I was pregnant after trying for 6 months. I had a miscarriage at 6 weeks. 
We are more than likely moving on to month 3 of TTC again now. I know that it takes normal, healthy couples 12-18 months to conceive. It took me 6 months to get a positive last time, so I know that this is a process. 
Obviously I'm ovulating or I wouldn't have gotten pregnant already so that's doesn't seem to be a huge issue or necessarily my first concern. I get extremely clear OPKs every month too which I know doesn't confirm ovulation but just hint that it "should" be coming within 12-36 hours. 
I have been taking prenatal vitamins for months and months. I use OPKs. I have just started using PreSeed this cycle (AF due 4/15). I am wondering if I don't produce enough cm for sperm to live or if maybe I have hostile cm that is killing sperm off before they can reach the egg. Might sound crazy but it's just one of those "what if?" questions.
So basically I have a couple of questions.
 
1. What are infertility issues that you have experienced or someone you know has experienced?
2. Should I got to the Dr. about my concerns after 6 months of TTC the second time (12 months total), or do I go when I hit a year of TTC the second time which would be 18 months total? 
Any advice is welcomed and appreciated. I know that I'm not out yet for this month, and I'm not "jumping the gun" on infertility or anything like that. It's more of me being curious and wanting the knowledge of other experiences with this as well.