Poor size marigold plants?

I have been planting the Lady (First Lay) series of marigolds for over 20 years and every year they have grown 20 - 24 inches tall and approximately 12 to 16 inches wide. Last year the plants grew less than 12 inches tall and about 8 - 10 inches wide. This year they also look very small about 8 inches tall and not looking to grow any taller. I raise my plants indoors from fresh purchased seed every year and they are normally 6 inches tall and harden off with one blosom each when I plant them the 1st week in June after the frost here in northeastern PA. They get a nice constant supply of water without over watering as they are under a slight overhang. They currently get 7 hours of sun as they have over the last 20 years. I use Peters Blosom Booster for fertilization as I have done so over the last 20 years. I added compost this year and none last year so that doesn't seem to contribute to the small size.
Any suggestions as to how I can get my marigolds to grow to the 20 -24 inch height as I have done so in the past?
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