You may think both sides have the same soil but the side that is failing to thrive may have a different ph value. If it is slightly more acidic than the other side it can make a difference. I see a tree in your picture on that side. What kind of tree is it? Some tree leaves like oak or conifers can acidify the soil they decompose in

Looking at your photo it seems that the azalea in the left bed is much bigger than the one on the left and that could be a tell tale sign that that bed is more acidic. Azaleas prefer acidic soil. You can sweeten the soil with agricultural lime or acidify it with peat moss. If you acidify the whole bed try dwarf zinnias. They will tolerate acidic soil. My suggestion is the Magellen series. They have 4 inch flowers on 12 to 15 inch plants and are a favorite of mine

I think of Catharanthus as a sun loving plant. Perhaps that side is shadier than you think.

Catharanthus has to have very well drained soils and fails to thrive in a rich, peaty site. If there's a tree sharing the same soil as the bad side, I'd suspect that roots or allelopathy are to blame.

What about irrigation? This is a plant that cannot tolerate overly wet soils.

Tree leaves really have very little effect on soil pH, neither do pine needles. But unless your existing soil is absolutely clayey or sandy, I'd not amend with peat/mushroom compost again, especially not for the Catharanthus.

I'd not add lime or soil acidifier without having a soil test first.

So! How many hours of full sun does this area receive?

if it were me... next year.. instead of all one thing... a monoculture ...

i would plant 6 or 8 different things ... AND SEE WHAT DOES BETTER .. in that specific spot ...

in other words.. plant a test garden ...

i hesitate to buy into the pH theory ... its possible ... i find it not highly probable ... unless all nest years test plants fail on that one side.. then you have the answer ...

ken

ps: NEVER plant a monoculture.. one bad problem comes along.. and you lose it all .. not that big a deal with annuals.. but it still could be a problem ...

thanks for the comments. I think I will take samples of soil and have them tested. I plan on planting bulbs next month as well and I like the idea of planting several varieties to see what it likes it there. I do think ph is playing a role because on the bad side the petunia is doing well but the good sides petunia is not. they like a different ph that the vinca. sunrise both sides get morning sun to midafternoon then are in dappled shade the rest of the day.

Interestingly the white petunias are thriving on the opposite side from the Catharanthus. I wonder what that tells us?