Success may be based on factors such as:

-- Breed --

Fair or not, certain bully breeds or mixes with bully breeds are not easy to find foster homes for.  Also, labs or other more common breeds or mixes, may have a more difficult time than others.

-- If your dog is a purebred or close to it --

Whether it has "papers" or not, purebreds are easier to find rescues for, as some rescues will not take mixes.

-- Temperment --

Whether it gets along with other dogs or cats or kids, or just its demeanor in general, has an influence on whether a rescue will take the dog or not.

-- Age --

Believe it or not, seniors sometimes have an easier time making it into purebred rescues because of the love of those people for their breed, and the fact that seniors are usually more laid back and easy to foster.

-- Size --

Small and medium sized dogs sometimes have an easier time getting into mixed breed rescues

-- Other factors --

Certain times of the year, rescues may be fuller than others.  Even if your dog needs medical attention, sometimes the pitty factor can help your dog get into rescue and get the medical attention it needs.  There are a variety of other factors, and sometimes it just depends on the urgency of getting a dog help versus having the animal put down.