Why would a female dog not get pregant with a tye?she has had litters b4 and my stud is proven

When dogs tie it is like a 99% chance of pregnancy, she may have fallen into that 1%.....also if she was not in heat, or she was in her last days of being in heat, then her eggs were not ready to receive. Her not being pregnant after being tied does not mean anything is wrong with her or ur stud (unless u notice anything different about her) if she is acting the same, eating, drinking etc...then she is fine....it. just wasn't the right timing that's all.