taking for granted that you have already checked your connections throughly, first thing you should try is changing the string from "hello" to "HELLO" since the display can't display lowercase characters XD. Another curios thing about this display is that every minute the screen is cleared of all symbols and characters (this is done by the display itself, not by my library, and i don't know if it's possible to avoid this), so since your sketch includes a 2s delay, the only symbol shown goes away for up to 2 seconds from time to time. BTW, all other symbols are working too? if you assign i=1 you see the dolby c and so on?
hope this was helpful