The boiler with a coil heats only part of the water from the top

Good morning
I am asking for your help.
I have central heating installations, house heating and a boiler with a horizontal coil, the water in the tank heats up only a little (by checking it, about 5 cm from the top, the tank is hot and the rest is winter and the result is one shower and the end of hot water). There is no air in the installation (input and output correspond to the temperature on the stove). Whether I smoke shorter or longer with a temperature of 90 on the stove, the effect is the same. Should the boiler be installed exactly horizontally or at an angle? It happens from the very beginning, not even once did the tank be completely warm? I am asking for advice and help with what it may be caused.
best regards

Pictures and installation diagram, please. The boiler should be horizontal.

What a colleague describes is impossible. If the return from the coil has a specified high temperature, the water in the DHW at this connector should be the same. Depending on the shape of the coil, a different part of the tank will be heated, while a horizontal double-jacket tank and a vertical one with a coil are the best. If the return from the domestic hot water is hot and there is little hot water, I would see the poor coil as the fault.

If there is something wrong with the coil, it remains to replace the boiler?

Have a colleague answer question first. colleague Gersik, give the exchanger model and see what can be done.

If the boiler has coils as shown in the picture and is installed the other way around, it will appear exactly as the author of the post describes.