we had only one or at most 3 relay there. all of relays i mentioned can support dms well. for example when i use jellyfish immortal version on 0xchat between 2 accounts on same device (my own phone) it works well. but between two devices it won't work.

afaik dms are simple in relay side same as other stuff, the most complex part is performed on client side.

i can imagine 1 case for it:

1. between relay or clients there is one of them which can't make or response to queries about dms well.

for 0xchat relay specifically i think its banned in iran which make it more difficult to get it into work.