Watermelons are thought to have originated in the Kalahari Desert. This is a latitude of 25 degrees South. Michigan is about 45N. So It made me wonder, since they were grown historically in latitudes similiar to yours. I stole most of what is below, but wanted to understand the context of your issue. It did not show up at first as I started by looking up latitudes the plant thrived in historically. This climate zone picture was enlightening. It was actually on a michigan related link:
https://michigan-weather-center.org/climate-zones
You are in a cool summer D climate in Michigan. This is the main problem, the plant chemistry needs the energy of the heat. In short it is stalled out.
D – Moist Continental Mid-latitude Climates
Moist continental mid-latitude climates have warm to cool summers and cold winters. The location of these climates is poleward of the
“C” climates. The average temperature of the warmest month is greater than 50°F (10°C), while the coldest month is less than -22°F (-30°C). Winters are severe with snowstorms, strong winds, and bitter cold from Continental Polar or Arctic air masses.
In 1850, explorer David Livingston found them growing wild in this area in southern
Africa. The Kalahari Desert covers 70% of Botswana, and parts of Zimbabwe, Namibia, and South Africa. The first recorded watermelon harvest occurred nearly 5,000 years ago in Egypt and was depicted in Egyptian hieroglyphics on the walls of their ancient buildings.
From Egypt, it is believed the watermelon spread throughout countries along the Mediterranean Sea (45N at it's north-most area) by way of merchant ships.
Watermelon spread through the rest of Europe via the Moors during the
13th century.
There is a funny story about a Roman leader and a water-
melon. A watermelon was once thrown at Roman Governor Demosthenes during a political debate. Placing the watermelon upon his head, he thanked the thrower for providing him with a helmet to wear as he
fought Philip of Macedonia. (Rome's latitude is 42N.) Maybe they imported them from their south, I do not know.
Southern food historian, John Egerton, believes watermelon made its way to the United States with African slaves. Food historian John Martin Taylor
says that early Greek settlers brought the method of pickling watermelon with them to Charleston, South Carolina (33N).
Watermelon thrive in U.S. Department of Agriculture plant hardiness zones 3 through 11. This range includes most of the United States, but heat, the growing season length and planting conditions are also important for success with growing watermelons. If planting watermelons in cooler climates, start the seeds indoors and select a smaller variety with a shorter growing season, suggests Harvest to Table.
The ideal temperature range for growing the sweetest watermelons is between 70 and 90 degrees Fahrenheit. Watermelons need heat throughout the 2 1/2 to 4 months they are maturing on the vine, from fruit set to harvest. This allows them to develop flavor and sweetness. Cool temperatures, below 50 degrees Fahrenheit, will cause the watermelon to taste bland.
Watermelons have a long growing season, from planting to harvest, and during this time, they should not experience a frost, which could kill the plant. Short-season varieties are available if you do not have warm temperatures long enough to produce a fully sweet melon. Short-seasoned varieties may also be started early in the spring for areas with exceptionally hot summers, because temperatures above 90 degrees Fahrenheit will cause flowers to fall from the plant, preventing fruit from setting.
The conditions for growing watermelons include well-drained
soil with a light, loamy texture and plenty of sun and water. As 95 percent of a watermelon's weight is water, you need to water the plants with 1 to 2 inches of water each week while the vines and flowers are growing and at fruit set. Reducing watering as the fruit grows helps to concentrate the sugars in the fruit, making it taste sweeter, notes The Old Farmer's Almanac. Wait until the watermelon produces a dull thumping sound when tapped with a finger, the bottom has turned a creamy yellow color and the vine has dried and withered. These are indications that the watermelon has reached its peak ripeness and it will not ripen further, on or off the vine.