Why has about 75 of your red verbena died after planting it this spring you were very careful to take good care of it?

Most Verbenas are not frost-hardy, some of them such as V. Officinale will tolerate frost up to -6/7 Degr. Celsius. V. Bonariensis will survive in UK outdoors if in a sheltered spot and should be ok down to -3 Degr Celsius.

So it depends on the shelter your 25% have had.