Since you have the big one for comparison, I guess they're not all suffering the same problem, which lets out my main suspect: cold damage. Sometimes lack of nutrients, especially potash, will do that. Viruses can also do that, though I don't remember which ones. What does the rest of the plant look like?

I did not take that picture but all my periwinkles look like the small one and i live in the tropics(although it has been cooler due to the nonstop rain). Could it simply be that the plants are old?

I wouldn't call them old unless they were more than 4 years old. They may need a heavy pruning and some fertilizer to rejuvenate them. I would know more if I could see what the plants look like, or get a detailed description of the color and size of the leaves and stems.

Foolishly envying your rain!
Kevin : )

Sorry I took so long. Lost track of things. I cut the plant down hoping it will correct the problem but same thing.

{{gwi:25841}}

Whoa! That's a really shady spot, from the looks of it. Madagascar periwinkle needs a minimum of 6 hours of direct sun a day for normal flower development. The lower leaves are also indicating some kind of nutrient deficiency, possibly a combination of nitrogen, manganese, and zinc. Sorry, tropicsnut, Madagascar periwinkles need more sun than Caladiums can stand. Looking again, magnesium deficiency is also a possibility.

Kevin : )

I don't think it's the sun because i have other periwinkles out in the open with the same problems. I'll treat for magnesium deficiency. Hopefully it gets better. :)