Depends on how your clients is selecting the relays to transmit those DMs...
With noStrudel.ninja (browser) and Nostur (iOS app) DMs are working quite fine in my case 😄
With that said, I prefer using SimpleX for private messaging 😉
Discussion
Same here. But some people write to me here, so I just try to find solution…
My nostr:npub12vkcxr0luzwp8e673v29eqjhrr7p9vqq8asav85swaepclllj09sylpugg also behaves so strangely… 
nostr:npub16c0nh3dnadzqpm76uctf5hqhe2lny344zsmpm6feee9p5rdxaa9q586nvr Any suggestions to improve her experience?
Well...to be honest I stopped using Primal already more than 6 months ago because I had a lot of issues with it as a Nostr 'superuser' (I use / test a lot of different Nostr clients)
So what is alternative? nostr:npub18m76awca3y37hkvuneavuw6pjj4525fw90necxmadrvjg0sdy6qsngq955 works ok, but again frequent problems with DMs, or no ability to upload videos.