Plants that have to be brought in to avoid frost are the hardest to look after. Any bugs on them which have perhaps been held in check by natural predators will often multiply when indoors as will any diseases. As an example my P.Adularia when outdoors was fine but when in for its first winter got a very bad scale insect infestation & was very poorly....but once outside again the next year made full recovery without any treatment. I also often have leaf roller problems. If you have difficulty over-wintering your plants try to analyse which of the variables affecting them need to be changed:-
Even if you get a lot right your plants will still lose leaves & drop buds. My plants, both indoors & out tend to look a mess for half the year & spectacular for the other half. I work on the principle of trying just to keep the plants alive until light levels increase in spring & then off they go, but watch for a warm spell followed by cold which can kill them if growth restarted.
A tip for succeesful overwintering indoors comes from Charlie Pridham of Roseland Nurseries. He keeps all his Passiflora in 10''-12'' pots. Each January he pulls them out of the pot, knocks all the soil off them, cuts them back & repots with fresh soil. He has kept some original plants alive since the 1980's with this method.
I guess some of us are pushing the plants to their limits - I have lost a lot of plants over the years - sometimes I feel more like a plant murderer rather than a grower! They can be very difficult to help - even the experts like John Vanderplank are often faced with a plant that is slowly dying & just don't know which variable to adjust to bring it back to health. The most important factors without doubt are to keep the plant as dry as possible, water from below & use a fan for air circulation.