You chose a sick plant
When buying your seedlings, look for the healthiest plant available to give your garden the best chance of thriving. That means there should not be one bug or egg on it (as one aphid means there are plenty more invisible ones on the plant).
You planted herbs in the wrong spot
Did you think about the growing conditions your herbs need? Herbs from Mediterranean areas, like lavender and thyme, are best grown in terracotta pots; while Asian herbs, like Vietnamese mint, will need more water and prefer plastic pots. Consider the amount of sun and shade each plant needs to set up a successful herb garden.
You didn't prune regularly
To keep producing healthy leaves, herbs need to be cut back regularly. If you don't prune, the stems will grow tall and leaves will dry and fall from the plant, leaving you with a bunch of leaf-less stems. Pruning is a win-win situation: the plant stays in the growth stage and you get to use the fragrant leaves in the kitchen!
You overcrowded your herb garden
More is not more when it comes to a healthy herb garden. You want to give each plant enough space to grow fully and establish strong root systems to help it survive winter and beyond. Be sure to read the tag before planting to find out the estimated height and width of the fully-grown herb.
You sprayed the herbs with chemicals
Never put chemicals onto your edible garden. Instead, prevention is the best cure to keep invading weeds and insects at bay. Always use natural fertilisers and look for products which are labelled as safe for edibles.
You let flowers turn into seeds
Don't be seduced by pretty flowers on your herbs. Remove flowers ASAP otherwise they will turn to seed and the whole plant will die off for the season.
You didn't water properly
A little water each day goes a long way to growing productive herbs. Add water to the soil at the base of the plant; never water the leaves as this could lead to mildew and disease. Make it part of your morning routine as the water will be able to soak into the root system with little evaporation.
You didn't use mulch
Mulch is a must for growing impressive herbs as it will keep weeds at bay. It will also help keep moisture in the soil, making your herb garden more water efficient.
You didn't fertilise
Herbs are going to need a lot of energy to keep growing as you harvest so be sure to give it plenty of fertiliser. Compost tea applied to the soil is ideal. Otherwise, look for another fertiliser suitable for edible plants.
You didn't protect your herbs
Do you know what the healthy herb should look like? When leaves and stems start to fade, curl, turn brown or look damaged it is time to act. Turn to organic methods of pest control so your health won't be at risk.