Just for starters and in no particular order........slugs and snails, next door's children's footballs, unseasonal weather, council green waste collections not starting until March when the main tidying-up time is February, not enough rain in summer, unpredictable variability in peat free compost (between bags of the same brand/type).
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.