Hello and thank you to everyone who helped me with my potting soil question a couple of months ago. I wish I could say all my problems are solved, but that sadly

isn't the case. I have been using HVH fertilizer @ 1/2 tsp per gallon, Super Nova 1/2 tsp per gallon, and Hibiscus Booster 1/2 tsp per gallon, with every watering. I have also been VERY careful about how much water I give. They all seemed to respond great at first. Started to get new leaves, new blooms and new branches coming up from the bottom of the plant. About two weeks ago they all started to go down hill again. The first thing to go was the new growth. It started to wilt and eventually the new stems died. One of the plants that was starting to do so well now sits there, nearly a bare stick. Three of the others are almost as bad. They look like they have a different problem though. Their leaves turn brown starting at the edges and then eventually turn completely brown and fall off. The interesting thing about these plants is that they are all three the same variety ...Stolen Kiss .... First Kiss ..... and Hugs and Kisses. It looks a lot to me like fertilizer burn, but if that is the case, why wouldn't all the plants be reacting the same?
Also, I live in central Florida and spring is rapidly approaching. I want to repot using a better quality soil in early March. I think (thanks to all the help I got from all of you on this forum) I know what potting mixture to use, but I don't know "how" to repot. Should I just go up a pot size and fill in the space with the new potting mix, or should I try to remove the old soil from the roots? Should I even try to repot these sick plants or just leave them alone until I can get them stabilized?
Thanks again for all your help. I would probably give up if I didn't have this resource.
Kathleen