It's an easy mistake to make, but do avoid removing the lid to take a peek, unless your recipe tells you to do so in order to add an ingredient.
If you add too much liquid to your recipe, it will become very hot and cause excess condensation, which will drip from the lid back into your dish and make the recipe too watery.
If your recipe includes wine, a slow cooker doesn't get hot enough to boil off the alcohol, leaving a strong taste.
If your recipe includes dairy, such as cream or yogurt, add them right at the end, long enough for them to heat through fully, around 30 minutes.
Browning meat adds texture, colour and flavour, so remember to brown the meat over a high heat first, whether on the hob or in your slow cooker.
When you cook meat and vegetables, they'll produce more liquid than you started with, so ensure your cooker is large enough to accommodate it.
When cooking a dish for such a long time, you may lose the flavour of any dried herbs you have added, especially if using a recipe which hasn't been written specifically for a slow cooker.
Vegetables which are relatively quick to cook on the hob should be added towards the end of the cooking time in a slow cooker, around 30 minutes.
This problem may be down to a couple of issues. Firstly, don't forget to set the timer! It could also be your recipe. Some slow-cooked recipes call for a certain amount of time on low, then extra time on high.