Most of them tend to be anxious in carrier, especially it is their first time and some may not be comfortable staying in for hours. Training is required to keep them calm and get use to the carrier. You can place the carrier in the house treat it like a safe zone, put treats/toys in if you have to lure them in for the first few time. Treat this carrier like a safe zone, every time they want to escape something like nail trimming or after getting scolding they can hide in this carrier and you let he/she escape fearful incident. If they get comfortable going in voluntarily and staying, try zipping for a short time. See if they can settle in carrier without struggling to get out. Try a few more treats/toy luring if they are still anxious. If you can keep the carrier zipped try playing some aircraft sound to get them used to the noise as well.
