Not including the plec and siamese fighter, you have at least 64inchs of fish(guppys=1inch, mollys 3inchs) in your tank making it realy overstocked, you should only have 1inch of fish per gal as a rule of thumb, and you have at least twice that much if we include the plec and siamese fighter...
Im not sure if your water test results are accurate, but i strongly suggest you start using dechlorinator as so your tank can cycle properly so there is less enviromental stress on the fish. The reason why some of your fish may have died when you last used it is that it started to cause your tank to cycle but you had too many fish for the cycle to handle- there's more about cycling tanks in the beginners section of the site on the stickys at the top of the section. But if you want to have healthy and happy fish you need to cycle your tank.
If you want your fish to reproduce and produce healthy offspring, i strongly suggest you rehome 9 of your mollys and 20 of the guppys; do you know what type of plec you have?
Im not trying to be harsh here but only state the facts, but your tank will be much better off with alot less fish and i know others will back me up on this.
Your fish havn't been having fry recently because of the amount of fish you have in the tank and their bioload and the stress each others company is causing to another and the likelhood that most of them are eating each others fry.