I will try to lay this out in simplistic for.
Breed Pug

Male 1 was bred to female 1 = 6 pups all lived. Repeat breeding 1 year later miscarried whole litter 4 wks in.

Male 1 bred to female 2 = 7 pups all lived repeat breeding 1 year later miscarried 2 wks in

Male 1 bred to female 3 = 4 pups all lived   female 3 bred 1 year later to Male 2.  Miscarry 3 wks in

Male 1 bred to female 4  = 3 pups. 2 live

Male 1 bred to female 5 = 2 pups 1 lives

[Admin: breed allowed as it may be relevant]

https://www.petwave.com/Dogs/Health/Abortion/Causes.aspx

There are other websites.  I suggest you do have a problem in there.

Unfortunately I think there is a rise in CHV in the US and currently the vaccine is not available there.

I think you're in the US?  Has the stud been tested recently for brucellosis? 

Otherwise I would suspect CHV and would not use the stud for several months until his body has had a chance to clear the active infection - that way it will not get passed on at mating as may be happening now...