I can't speak to the exact circumstances in your garden but too much organic matter can have the effect of "tying up" nutrients in the organic matter in a way t

I can't speak to the exact circumstances in your garden but too much organic matter can have the effect of "tying up" nutrients in the organic matter in a way that makes them inaccessible to crops - if you grow to large a microbial biome in the organic matter they immobilise nitrogen.. I am not an expert on this so I only understand it in broad strokes. I am a bit of gardener and my anecdotal advice is - I don't know what sense you are using the word "mulch" here so I am not sure about that but too much compost can be an issue if you are not letting it break down long enough and not incorporating into existing soils well enough. Too much compost will grow wicked-hot radishes, but your leafy greens will be ugly and taste like garbage in my experience. The thing about gardening is it is all small sample sizes.. it ain't science and it is hard to know what's up without a good old fashioned site visit!