Then you may not be getting the 1080p encoding. They need to test your connection to their servers as being something like 140% of the average speed of the encoding--it has to be faster than the speed of the encoding to stay ahead of it, keeping a buffer full in case the speed should dip below the encoding speed for a while. If their 1080p encoding was something on the order of 5.1 Mbps, then the player would have to see at least 7.2 Mbps available bandwidth on your connection (Netflix has stated an 8 Mpbs requirement for 1080p).
It's sad that no known player other than the PS3 will tell you what encoding you're playing (the Xbox does have a scrub bar that you can bring up with a 3 bar meter and the letters "HD"). The Roku has a debug mode that used to at least tell you what bandwidth it detected when you started it but it doesn't work in the Roku 2 Netflix player (still works in Amazon).