why is it so much harder to get pregnant the second time?

Alina
I had my first baby around 7 months ago, when I fell pregnant last year , it was very quick and unexpected, making love with my partner only once. Once I saw my baby for the first time I fell in love and I just knew that being a mother was for me. 
Since my baby arrived 7 months ago , I haven't used protection at all as I wish to get pregnant again. I know it might sound too soon for you but I feel I can do it and I don't have much family members of my own so having my own family is very important to me. 
So I've been trying to get pregnant for about 5-6 months , making love a few days before the fertile window and during the fertile window every month and I'm still not pregnant !!😳
My question is , why is it so hard the second time around to get pregnant , especially when I've made love during monitored fertile days ? 
I am very grateful to God even for one baby so don't get me wrong but my heart is really craving for a bigger family. I am also 27 so my dream of having 5 children might be running out...