According to the textbook context, shareholders may fail to pay one or more instalments on shares allotted to them, specifically allotment money and/or call money. In such circumstances, the company has the authority to forfeit shares of the defaulters. The provisions in Table F authorise the directors to forfeit shares for non-payment of calls made. The other options (failure to attend meetings, shares pledged as security, or failure to repay bank loan) are not mentioned as valid grounds for forfeiture in the provided context.