Question. I had the MMR shot has a kid. they checked my blood for antibodies when i was pregnant…

Question. I had the MMR shot has a kid. they checked my blood for antibodies when i was pregnant with my now 3 year old and told me i had nope for it so got another shot after he was born. They checked for antibodies again with this pregnancy and said i still have have any antibodies and need another shot after i give birth. has this happened to anyone else? why wouldnt have an antibodies after i have been vaccinated multiple times?