It is completely normal to lose a shrimp or two within 2 weeks of shipping. Sometimes they just can't handle the stress of the move, no matter how well you acclimate them and treat them.
What is the behavior of the shrimps you have now? Are they circling around the tank restlessly or are they hanging out and grazing on plants, moving around whenever they're bored of their area? Are there a few hanging out at the top of the water in the corners?
I strongly recommend not changing the water for around two weeks after you receive new shrimp. They're extremely weakened and stressed from shipping and the most important thing to help their stress levels is stability. It really does take a while for them to get completely comfortable. Keep the temp, water, lighting consistent and stable during this period. I also don't feed for at least a few days after new shrimps come in. After two weeks, resume weekly 10% water changes.
Guppy grass is great for shrimp tanks. Do you have any other plants in there? If you can get a hold of hornwort, I'd put some in. It does wonders to reduce nitrates to zero. I think even low nitrate levels stress out shrimp and is one of the causes of those "unknown cause" deaths. Hornwort and other floating plants soak up nitrates better than most other plants.
Indian almond leaves do wonders for the stress levels of shrimp. Adding a leaf in the tank may help as well.
+1 on adding cuttlebone or other micronutrients to the tank (tourmaline, montmorillonite, etc.)
