Westpac has used cash-back offers, interest rate discounts and commission bonuses for brokers in a bid to lift home loan market share this year but nothing seems to be working. Growth in its mortgage book is stuck at about half system growth.The country's second biggest mortgage lender is offering a 90 basis point discount off its advertised standard variable rate until the end of November and its subsidiary St George Bank is offering a 100 bps discount.It has a $1500 cash-back offer for home buyers wanting to take out loans greater than $500,000.And a 10 bps bonus offer for brokers, which was tied to volume targets, expired on September 30.Despite all this, the latest Australian Prudential Regulation Authority figures show that Westpac mortgage book grew by 0.2 per cent in August, compared with system growth of 0.4 per cent. Over the three months to August, its book grew by 0.9 per cent, compared with system growth of 1.5 per cent. And over the 12 months to August, its book grew by 1.8 per cent, compared with system growth 4.6 per cent.