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IN most cases, you can grow medium to low light garden plants, and not see a difference in growth, or you may even see better growth. That is not to say that you can't grow plants with high light requirements, but that they may not do as well as a plant that is more adapted.

This list can help you to find plants suited for the area:

Vegetables:

Lettuce
Spinach
Kale
Radishes
Beets
Swiss Chard
Arugula
Endive
Mizuna
Mustard Greens
Fruits:

Strawberries
Currants
Gooseberries
Rhubarb
Blueberries
Blackberries
Raspberries
Cranberries
Figs
Peaches (Certain varieties suitable for partial shade)

Now, testing soil will be helpful. Testing for pH and nutrient imbalances will remove all doubt, there.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-nutrients-everything-you-need-to-know.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm

It is best to hot compost manure first before using it. Fresh manure can burn your plants as well as introduce harmful pathogens and weed seeds. You could let it sit through the summer and then start a fall garden. But, without the benefit of hot composting, you run the risk of weed seeds germinating and bacteria such as E. coli persisting. Since you already planted squash, you can go ahead and use it since you will be cooking the squash. Be sure to wash it well before preparing.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/using-fresh-manure-in-gardens.htm

https://www.gardeningknowhow.com/composting/manures/the-benefits-of-manure-in-your-garden.htm

https://extension.umaine.edu/publications/2510e/

You can flush your plants with more water if you are worried about a chemical transference.

Here is information on soil testing:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

You also can buy soil test kits in home improvement-type stores.

Ultimately, there are only a handful of things that can cause this. Overcrowding, excess of nitrogen, pH issues, and compacted soils.Once you can narrow these down, then you can begin to correct the issue.

https://www.gardeningknowhow.com/edible/vegetables/radish/radish-does-not-form-bulbs.htm

Water is most important. The soil should stay moist but not saturated for germination to occur. Also, were the seeds planted at the right depth or left uncovered if they need light to germinate? Were the seeds fresh, or if not, stored properly? Is it possible the birds ate the seeds?

Here are more factors related to germination:

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/seed-germination-requirements.htm