When you called strtok at line 3 it replaced the ':' in line with a '\0' (because that is how it works). That reduces the string in line to "end" which does not contain ".word" and leaves tptr pointing at line. If you called strtok a second time then tptr will be updated to point at the rest of the string " .word 5" which does contain ".word".
On the whole I would recomend not using strtok at all because of this destruction of data and because it is not re-entrant. It is relatively easy to parse the strring yourself.