Is a towing company responsible for missing items in a car that they towed?

If your car was volunatarily towed (e.g. it broke down and commissioned the tow) and if the company prominently, before you hired them, made you aware in some fashion (including by prominent signs on their truck, or information on the paperwork) that they are not responsible for missing personal belongings and you went ahead and had them tow anyway, then you would not be able to press a claim. You hired them subject to a disclaimer of liability.

However, otherwise--including if your car was towed involuntarily (e.g. for being parked in the wrong place)--then you may demand that they compensate you for the missing items. They normally would be responsible for items in the car, except in the situation outlined above. If the items are worth it and they refuse to pay, you could even sue them.