The last one is primal, right? It has haters for a reason 🤭
Apart from that it's probably DNS... I mean relays. But don't ask me how to fix it, I wish I knew. The outbox model seems to be promising in that regard, but I don't know if there's an iOS client with outbox model and frankly I still encounter issues. Wisp seems to be the most reliable, but usually I switch between the clients and sometimes I notice that also wisp missed some notes.