can occur for a number of reasons and blockage of the fallopian tubes is one of them. This blockage usually occurs as a result of infection or any other scenario that can cause scarring ( previous surgery, congenital problems,inflammation).
A previous spontaneous abortion
does not usually cause blockage of the tubes.
Problems conceiving can be related to male and female factors and in alot of couples it is a combination of both.
In females there can be issues in any part of the reproductive system - the cervix
, the uterus
, the fallopian tubes, ovarian factors
In males there could be problems with sperm production or transport.
To determine the cause your wife should consider visiting a doctor who will take a relevant history, examine her generally and conduct a gynae exam. Investigations can be conducted to determine whether a viable egg is being released, to detect any problems with the cervix and its mucus, to determine if there is any blockage of the tubes and any issues with the uterus.
In men, sperm analysis is done to check the sperm.
I hope this helps