Answer: Based on your description I am not certain what is happening. If the mulch were causing the problem then the other plants would also show signs of stress, so I would suspect possibly poor drainage in the particular area where they are planted so that they are being flooded, or else possibly colder soil conditions. The other plants you mention are cool season lovers, and grow fine in cool weather and cooler soil. The peppers and canteloupe however need warm temperatures and warm soil to grow well -- and if applied early, the mulch might be keeping the soil too cool. Finally, you might want to check on your soil's pH and fertility by running some basic soil tests -- that will help you in terms of knowing for sure about the effects of your amendments as well as how much to add (or not add) in the future.
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