I have no idea why you were told April was "too late in the year" to plant - that makes no sense. However, true pampas grass, Cortaderia selloana, is not considered reliably hardy in zones lower than 7. Some folks have had luck with it surviving in colder climates, but it is certainly not a given. And it is an evergreen grass - if yours is not apparent now, then either it did not winter over or it is not true pampas (there are other grasses that are also called by this common name in error). If nothing shows up by later in the spring - like April!! - then I'd consider replacing, perhaps with a hardier choice.