You need at least 6ml of the syrup (based on US packaging, which at least used to and may still differ from Europe's) per roll. Dilution H is very dilute, containing only ~4.7ml of syrup in 300ml of mix, so you probably exhausted the developer, resulting in underdevelopment. Either use more solution, or a different dilution like B. The trouble with A and B is that developing times can be very short, so if your tank is limited to 300ml you might want to use D or E, especially in warm conditions.
HC-110 is a nice developer, you should be able to get it to work. Google "HC-110 Covington" and you will find a very comprehensive resource page. It is also available in German translation if that is easier for you.