Thanks for your question.  All indications point towards slugs.  Specifically, how did you treat for them?

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For iron phosphate to disrupt the digestive tract of slugs (its mode of killing), it must first be eaten. Iron phosphate DOES NOT KILL ON CONTACT. Products containing iron phosphate also contain some attractive bait by which to entice slugs to eat the product. Iron phosphate can take as long as up to a week before the slug is killed. This implies that one may have to wait at least that amount of time before any significant decrease is seen in the slug population. Also repeated applications may be necessary.  In your pictures, I see the chard is very close to the ground and very close to the pellets containing the iron phosphate. It may be that given a choice between luscious chard leaves and rather inert pellets, slugs will prefer the former. It might have been more effective to have scattered the pellets around the chard in a circle and at a distance from the chard. If done in his way, slugs would first encounter the pellets before getting close to the chard. In other words, make the pellets a barrier. Hopefully they would then eat the pellets first and not be sidetracked by nearby chard leaves.

Note that another product intended for slugs and containing 1% iron phosphate, the application instructions are as follows (note the highlighted lines):

“Use the bait to protect any vegetables from slug and snail damage, including (but not limited to): Asparagus, Beans, Beets, Blackeyed peas, Broccoli, Brussels sprouts, Cabbage, Cantaloupe, Carrots, Cauliflower, Corn, Cucumbers, Eggplants, Garlic, Lettuce, Onions, Peas, Peppers, Potatoes, Radishes, Rutabagas, Spinach, Squash, Swiss chard, Tomatoes and Turnips. Scatter the bait around the perimeter of the vegetable plantings at the rate of 20.0 to 44.0 pounds per acre (0.5 to 1.0 pound per 1000 square feet) to provide a protective “barrier” for slugs and snails entering the vegetable plantings. If slugs or snails are inside the rows, then scatter the bait on the soil around the plants and between the rows.”

For the product you used, application instructions were (again note the highlighted lines):

"Scatter the bait around the perimeter of the vegetable plot at the rate of 0.5-1 lb per 1000 square feet, or for smaller areas, 0.15 oz, or 1 level teaspoon per square yard, to provide a protective “barrier” for slugs and snails entering the garden plot. If slugs or snails are inside the rows, then scatter the bait on the soil around the base of the plants and between the rows."

Also see the following describing how to deal with slugs (again note the importance of using baits as a protective barrier around the plants.:

https://extension.umn.edu/yard-and-garden-insects/slugs - baits-298065

https://www.dl-online.com/news/tiny-garden-slugs-cause-big-damage

The product that you used is highly effective provided it is applied as a barrier around plants and not interspersed with the plants.

Thanks for consulting us. We would appreciate receiving any comments or thoughts that you might have. Good Luck!!