When you compare strings it's case sensitive. That means if $grab doesn't have a starting capital B and the rest lower case, then the elseif statement will not be true.
I cleaned up your code and inserted some debugging lines for you:
echo "
grab => " . $grab;
elseif ($grab == "Blackjack"){
$check = mysql_query("SELECT * FROM bj WHERE bjowner= '0' AND location='$fetch->location'") or die(mysql_query());
$num_rows = mysql_num_rows($check);
if ($num_rows != 0){
if ($fetch->location == $location){
mysql_query("UPDATE bj SET bjowner='$username',bjmaxbet='0',bjearnings='0',bjminbet='0' WHERE location='$fetch->location'") or die(mysql_error());
echo "You got the Blackjack!";
}
}
}