We talk to more than 100 people a week, through our shows, residential and commercial visits, and online communications. One very common anxiety amongst customers is that they cannot yet see wildflowers they 'had' last year, or that they have sown the year prior.
Some wildflowers don't show themselves until May. It doesn't mean (yet!) that they have died or your planting/sowing was unsuccessful. It's just that wildflowers are like trees. Some come into leaf very early (March), and some come into leaf much later in the spring.
Here are some slow-coaches to appear. Reserve your panic for a few more weeks yet!
Yellow Rattle (sown) - you may not see me until mid-May
Hemp Agrimony
Purple Loosestrife
Common Agrimony
Wild Chicory
It is also true, that apparently 'dead or disappeared' perennial wills sometimes come back later in the year. If conditions have been bad (wet or dry in particular, as temperature is unlikely to kill your native wildflowers), your plant(s) MAY have gone into a form of hibernation. Don't write them off too quickly.