I have found Double Knockout to not be as vigorous as Knockout itself. Having said that, I wouldn't expect them to struggle to the extent that you are describing. Being on a slope, is it possible that the watering that you are doing is just running down the hill and not really penetrating down to their root system? I would dig down into the hole and make sure that they are getting enough water. You also might try digging them up and preparing a better hole for them. I dig a 2 x 2 x2 ft hole for all of my roses and generously mix the native soil with 1 part peat moss and 1 part composted cow manure. That give the rose a really rich but loose soil to get the root system established. It sounds like your roses are not getting enough nutrients so your fertilizer applications are probably not making it down to their root system.

tree oracle is giving excellent advice when he suggests your older roses are not getting enough nutrients because of the way they were planted or are being watered.

Another maybe off the wall possibility - could the landscaper have added large quantities of a slow-release fertilizer like Osmocote to the planting holes? If that is so, then the MiracleGro you added could be activating the slow-release fertilizer.

It sounds a little weird, but I once lost a favorite begonia houseplant when I added fish emulsion to soil containing Osmocote. I later read a report that some fertilizers can dissolve the coating that makes Osmocote slow to release. The effect is to release the fertilizer all at once which the plant cannot handle.

Something like that would explain why the newer roses are OK (they only got a normal dose of fertilizer).

Too much fertilizer can damage a plant as much or more than too little.

Claire