It could be, as you have surmised, that the other female dropped if you have leopard patterned fry developing. If your blue still looks pregnant, that is an ever stronger possibility.

That is why it is useful to isolate females. They should be separated rather before they are ready to drop so they are not injured or stressed when really loaded with almost full term youngsters.

Of course if your blue was housed with leopard males (in the shop?), she may drop fry showing leopard for some 6-7 batches. Newer matings may produce more from her line (if she is mated with a blue male) but doubts should exist.

Serious guppy breeders line breed their guppies. They separate their young as soon as they show gender signs (primarilly females showing the gravid spot). Virgins are only bred after they put on some growth and to selected males which show size, robustness, color and fins appropriate for the strain. Size of females obviously correlate with the number of fry. In the meantime food which could go into fry production will be used for growth of the female.

Stud books are kept on each pair. Often a breeder will keep 2-4 lines of a color form of guppy and back cross them every so many generations, still logging who is descended from
whom. This is why one breeder recommended dedicating about 16 tanks to one strain!

For more information do a search on Google or at the IFGA site for "line breeding" "line breeding guppies" "Tom Allen line breeding guppies" (Tom is a terrific speaker on the subject.) Other animal husbandry sites probably have similar sites. There are also fish conservation committee sites which could be useful.

As for that one female which is so thin. She could just be that way. :(

It could be disease or internal parasites (something in the Heximeda line?) or just inadequate diet. If possible include some
blackworms or white worms in her diet. The fats in worms and daphnias are very similar to the fats and oils in fish eggs (even those which are fertilized and hatch in livebearer's bodies).

Females of a number of species have been known to absorb their embryos when diet or something else is not right. That is disconcerting. We even know a lady who had that happen to her!

So a guppy not able to nourish 30 fry will not have them. Females well nourished will not only have more fry, but within the parameters of whatever species they are, the fry will be a little larger and healthier than those from less well conditioned females.

Is she in a tank where she can not compete effectively for food? Maybe put her by herself. This is where small tanks come in handy. :)

Try the "worm bowl" with her. Let us know how she responds to therapy.