What I did this year is to make my own potting soil for the large pots on my deck. I used 1/2 elcheapo potting soil with plant food in it from Wal-Mart and 1/2 peat moss. I used NO addition dry fertilizer. I drilled small holes in the bottoms of all my plastic pots to allow the water to drain freely. I put a smaller pot upsidedown inside the center of the planting pots to cut down on weight and use less soil as I use very large pots.

When I planted the impatiens (4 to a pot) I gave each plant a haircut by cutting the stalks of the bedding plant in half. I have kept the pots wet with morning waterings when no rain is in the forecast. And I have feed them each weekend since father's day with a spray of miracle grow.

I read some where, maybe at Jack Edens website, that impatiens need fresh soil and sould not be planted year after year in the same planting bed. As the salt from most fertilzers will hurt them.

So I think you are right fresh soil is the key

However, overwatering is typically the worse problem.

Finally, if the new gunieas do well but the regular ones does not it can also be too much hot afternoon sun.