I should point out that I'm a completely novice gardener 
I bought some plants recently from my local garden centre. They were hardy perennials, can't remember the names unfortunately but I was assured by a staff member that they would flower all summer then come back next year.
Within a week or two of planting in the garden, the flowers have pretty much gone. I have irrigation so they get plenty of water and garden faces South.
What's gone wrong?