Usually, detergent doors open optimally during wash cycle for maximizing the efficiency of both dishwasher tablets and cycle. If the detergent tablet is not dissolving completely, then there can be several reasons for that. If the dishwasher is not using enough hot water, then the detergent may not dissolve properly. Make sure the machine is running on longest and hottest cycle. If the hot water is taking time for reaching the sink faucet, then it will also take same time for reaching the dishwasher. So, this may result in a wash cycle that isn’t hot enough. It is better to run your water in kitchen sink before turning on the dishwasher. Apart from that, you should also make sure the dispenser cup is not blocking or clogging the unlocking mechanism. And lastly, make sure your fingers are not wet while placing the tablets into dispenser cup. This can make the tablet to stick to the dispenser rather than dissolving.