Your experience with breeding this fish is similar to mine. I along with many of my fellow GCCA club members would say that S. marie are awful holders. They either hold for 10 days and drop, or you go to catch them to strip them and they spit. I finally got fry after my 7th spawn. I got the group from a friend and they were already proven breeders. I caught the female and stripped her after 7 days (She actually spit in the net). I hatched the fry as follows in a 2.5 gallon tank (this is how I hatch all my eggs artificially with almost 100% success):
1. 78 degree F water
2. a small sponge filter with an airstone inside
3. small 6 watt heater
4. a lot of methylene blue. 3-4 drops per gallon
5. kept in relatively dark part of the fishroom
I had many eggs that were unfertiziled that went bad and know others who have had the same experience. I did daily 1/3 water changes, and turkey basted the bad eggs out of the container. Out of about 70 eggs, I got 7 fry, 10% ouch. When I stripped the fish, the next day 30-40 eggs were bad. Not good results, but I got my BAP.
I don't think you are doing anything wrong really, just these fish are poor holders and must be awful at fertilizing eggs. Also when you catch the fish and they spit in the net, they probably damage a good amount of the eggs.
Hopefully you get a good spawn soon!