There can be several reasons why Adam is not properly heated with an Atag boiler.
Boiler power too high
It is possible that the capacity of your boiler is too large. This causes the boiler to heat up much too quickly. If the power is too high, the boiler temperature will be much higher than the intended boiler temperature. The boiler cannot get rid of its heat fast enough. As a result, the boiler sees that the boiler temperature is above the intended boiler temperature and stops heating. This can already happen after a minute while hardly any water has been pumped around. Especially with zone heating often much less heat is required. You then have to set the power of your central heating boiler lower. High enough to properly heat your home and zones, but low enough so that it can start modulating earlier. Fortunately, with some boilers you can set the power.
Atag Q51C
A well-known very heavy boiler. Look in the installation manual how to set the following parameters:
Parameter 2: Set to 3 (main heating vlw, with radiators). It may be that this is too low (namely 50gr) then set it to 2 (default to 1).
Parameter 3: Set power lower. Max. 45kw, lower to, for example, 25kw (can go back to at least 9)
Parameter 14: Set gradient to 1 (power at low load).
Defective three-way valve
Does the heating stay on after you have used domestic hot water or showered? There may be a problem with the boiler's three-way valve. A three-way valve ensures that hot water goes to your heating or to hot tap water/shower. If there is a problem with the three-way valve, the valve can no longer move properly. There may be no heating or no hot water. What can also happen is that after hot tap water/shower has been used, hot water continues to flow, but then to your heating. This can sometimes stop by itself when the boiler has to turn on for heating, causing the valve to move. For example, the heating may have been turned on incorrectly for a few hours. In this case, it is recommended to contact your installer.
With the Atag 244C, the three-way valve is prone to failure, see also https://gathering.tweakers.net/forum/list_messages/1893589