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?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.