This is an odd thing, as there shouldn’t be any problem at all. There shouldn’t be a technical limit on how many elements you start and then save in a custom mood. However - I haven’t tried it with more than, say, twenty elements.
Does this also happen, if you add just one element? Also worth mentioning - some elements have a build in delay when being started by a mood. But I guess you have waited some time for the element to start?
Lastly - remember where you take that element from you include I to the custom mood. Once you start the custom mood, go back to the soundset you activated it in and check, if it is really playing (playing icon illuminated and ring around it proceeding…).