Why do chemical pregnancies happen?

Lovely • IG: MakingBabyDay TTC #4. Sept 2014 tubal reversal. Feb 2016 HSG: blocked tubes. May 2016 IVF: failed. Aug 2016 BFP turned to 4th loss
Is there a reason? I'm going through my second right now, I've been ttc for 4 months, does it mean something is wrong with me?