Answer: What an interesting project!
Several things may be happening. The heap or pile for a traditional hotbed was made in a deep pit so as to be insulated. The manure would be packed into it and water added to make it sort of ferment. Since yours is exposed to the cold air on all sides, ambiant temperature is working against you and will slow composting dramatically, especially in such a (relatively) thin layer of material. The older the manure, and the more it has been composted already to begin with, the less it will heat up again. So each cycle will be slower and colder. This is true in general composting as well. Finally, the addition of the carbon heavy materials would slow the heating as it is less "hot" than the manure as a material; you are balancing the "brown" leaves with the "green" manure.
At this point, you have some truly excellent material for a traditional compost pile and the composting process should pick up again when the weather warms up, although I wqould suggest piling it in a more cubic arrangement, about three feet on each side as a minimum.
Nowadays, we usually think of a hot bed as being a cold frame with bottom heat provided by possibly a heating cable (or lightbulbs) depending on its insulation and the outdoor temperatures and the plants we are trying to grow.
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