In order to validate a code, Mastock checks the presence (or absence) of any recent data (15 calendar days) in the provider historical data server. There are instances, for Yahoo or Google, where data are present in the intraday data server but not in the historical server. Technical analysis is based on historical data.
Another common cause of non recognition is the lack of suffix for non-US markets. For Yahoo, international markets stocks are coded with a suffix. For instance France Telecom code is FTE.PA where Royal Resource Ltd in Australia is coded ROY.AX. The link below gives a complete list of suffix to use. International Stock Codes for Yahoo