Help! TTC.

So after being told I was unable to have children most of my life, I found out 2 months after I got married that I was expecting! I now have a 16 month old boy and hubby and I have been trying for another for about 4 months. This is where it gets complicated.  Since having my 1st, I have lost 110 lbs, started being more active and have had very regular periods since April. Every month on the 21st I know to expect it.  So my question is... Why am I not pregnant yet? We have tracked ovulation and had sex every day for the 6 day window and everything.  I am not sure it will ever happen again : (