It sounds to me that the pressure from the way that you water is probably crushing them.
Try to water from below if it's possible...
Flood the area rather than do overhead watering.

In addition to watering them from below, were they root bound?
I just planted out a flat of annuals and was surprised to see very tight root masses circling each plant.
I had to tease out the roots on every plant.

This seems to be typical when alyssum is first planted. I always "fluff" out the roots, too, as they can get a little rootbound in the market packs. Don't overwater - just when the soil feels dry. All of a sudden, they'll hit their stride and perk up. Will take a few weeks after that for them to start mounding out and looking good. Be patient - alyssum is the one thing where you would rarely get bad plants.

Have always had good luck with the white, but have found the purple and rose will gradually disappear over the summer when warm/hot weather sets in.

"but have found the purple and rose will gradually disappear over the summer when warm/hot weather sets in."

Yes, I remember this happened to me once. Wonder why?

"but have found the purple and rose will gradually disappear over the summer when warm/hot weather sets in."

I don't know why, but I've given up on the non-white alyssum for that same reason. It looks so pretty in May, and I like that the fragrance is a little more subtle. But by July, it always looks like crud, and it doesn't bounce back when the weather moderates.

These are what I consider 'cool season' annuals. Like so many others in their plant family (Brassicaceae-like cabbage, broccoli, etc.), they simply fade out when the temperatures climb. When I lived in zone 8b SC, they were sold as winter annuals, along with the pansies. I kept pots of them on my patio all winter long.

ritug, you might try cutting your plants back rather severely. They may have become leggy in their plastic containers and simply need a good haircut. The new growth will be stocky and strong.

Thank you all so much for your replies. I think it could be the watering from overhead and they were a bit leggier compared to the second batch that I bought.
I did not open up the roots - Too late I guess now.
Also I planted them about 4-5 inches away - Is that fine or is it too close?
Also I live in Ontario, Canada and our summers do get to be quite hot - weather going upto almost 35-40 degrees celsius sometimes. Should I be ready to lose the pruple ones then?

Thanks again.

4-5" is okay and they'll be fine. They'll just grow together enmasse - which I think is a real nice look for alyssum.

Here on the shores of Lake Superior, 80 degrees Fahrenheit is considered unbearably hot (these folks did not spend long summers in DC where 90 felt like a break in the heat) and the purple or rose alyssum start petering out for me by July when we usually have beautiful sunny days in the mid to high 70's.

I always though the worst that would happen with the colored varieties is that they would revert to white. Maybe you'll have better luck with them than some of us did.

Thanks duluthinbloomz4
I hope you are right and I do get a nice look - spent a lot of money on these - also is the first year growing them. If not I will just go back to growing petunias, impatiens and zinnias.

Don't recall ever having the purple ones "peter out" -- and I've planted them for years. I have found, however, that come spring, while the seeds from the whites will have sprouted with wild abandon, the ones from the purples rarely do.