Chicken curry in a slow cooker turns out watery

My first question on here, so please go easy on me.

I have recently got a slow cooker and have been very pleased with the results, but that was using ready-made sachets of cooking mixes.

I am now trying to cook things properly from scratch but I think I'm missing something important.

This is the recipe I have used for chicken curry:-

• 4 chicken thighs


• 3 garlic cloves


• 2 large onions


• 1 green pepper


• 1 red pepper


• handful chopped spinach


• 100g mushrooms


• 400g tin chopped tomatoes


• 250ml chicken stock


• 3 heaped tsp of curry spice

I put everything in the slow cooker, give it a good stir, and then leave it to cook. It tastes nice, but the sauce is very watery.

I have used the same recipe in a pan and the sauce thickens after I simmer it for about 20 mins.

Do I need to change the recipe to use the slow cooker? Or am I doing something fundamentally wrong?