I bought a canon speedlite 550EX and tried to use it as a slave flash for my canon T1i camera but it's not firing. Is it compatible? Has anybody experienced this problem? Any help will go a long way. I am Frustrated.
The 550 can operate as a master or slave unit. But what are you using as a master unit? A T1i has no built-in capability to be a master. You either need another flash on the camera hot-shoe that can operate as a master OR one of Canon's speedlite "transmitters".
How are you trying to use it? Mounted on camera or off camera with a trigger setup of some sort? Your camera needs a transmitter in order to trigger it off camera. It doesn't have a built in trigger system to it. You'd need an ST-E2 trigger if you are trying to trigger it wirelessly.
Have you consulted the manual? http://cius.univie.ac.at/fileadmin/user_upload/inst_cius/ci/downloads/Canon_Speedlite550EX.pdf
FYI, it is only the latest Canon cameras that have the ability/feature of built-in flash master. The T4i, the 7D come to mind, maybe a couple others...but certainly not the T1i.