Have you fertilized your dahlias? They need food to bloom well so adding Osmocote in April will help them to grow fast. Another concern may be that covering with plastic and leaving it on too long in the spring. Remove the plastic in February or March as too long will damage the living soil creatures that help plants absorb nutrients. Bait for slugs early. Sometimes the new growth is mowed down by slugs or rabbits as it emerges, so the dahlias have a late start. Finally, when is the last time you divided your dahlias? Over time the tubers become weak without division. This fall dig up the tubers and cut them apart, store indoors and let the soil stay uncovered. Replant in late spring and remember to stake - I predict tall and beautiful dahlias in your future. Keep growing, Marianne Binetti