If you have a newline at the end of the snippet definition file, then YASnippet will add a newline when you expanding a snippet. Please don't add a newline at the end if you don't want it when you saving the snippet file.
Note some editors will automatically add a newline for you. In Emacs, if you set require-final-newline to t, it will add the final newline for you automatically.
First check the mode line to see if there's yas. If no, then try M-x yas/minor-mode-on to manually turn on yas/minor-mode and try to expand the snippet again. If it works, then, you can add the following code to your .emacs before loading YASnippet:
(setq yas/extra-mode-hooks '(the-major-mode))
where the-major-mode is the major mode in which yas/minor-mode isn't enabled by default.
If yas/minor-mode is on but the snippet still not expanded. Then try to see what command is bound to the TAB key: press C-h k and then press TAB. Emacs will show you the result.
You'll see a buffer prompted by Emacs saying that TAB runs the command .... Alternatively, you might see <tab> runs the command ..., note the difference between TAB and <tab> where the latter has priority. If you see <tab> bound to a command other than yas/expand, (e.g. in org-mode) you can try the following code to work around:
(add-hook 'org-mode-hook
'(lambda ()
(make-variable-buffer-local 'yas/trigger-key)
(setq yas/trigger-key [tab])))
replace org-mode-hook with the major mode hook you are dealing with (C-h m to see what major mode you are in).
If it says TAB but YASnippet still doesn't work, check your configuration and you may also ask for help on the discussion group. Don't forget to attach the information on what command is bound to TAB as well as the mode information (Can be obtained by C-h m).