Farmed Nexifera with a friend last night and for quite some time, we could only manage to get "good captures". We were also tranqing and capturing the Cryptilexes that spawn near the slime pool but I accidentally triggered the Nex drop while a Cryptilex was under it and it killed the Cryptilex. After tranqing the Nex, we got our first capture. I theorized that you need to "feed" the Nexifera to get the perfect capture. So far I've been able to replicate this three times.