The loss of pressure on the hot water can result from :
- an obstruction in the tube on the cold water inlet of the sanitary tank
- the presence of limestone or sand in the bottom of the sanitary tank

When unscrewing the sanitary connections; after lowering the pressure of the heating circuit to zero; you can probe with a rod through the hot water outlet the depth of the boiler in order to evaluate the fouling.

The depth is +- 80 cm. It is possible that there is a scale plug that has formed in the tube on the cold water inlet.
In this case :
- you remove the plunger from the top and clean or replace it.
- you chase it in the boiler and replace it

You can find spare parts at sanitary and heating wholesalers near you.