My current game I had a market share of 11% in England and France in 1905, selling around 5000 cars a month. So I guess there is no bug.
But I usually try to sell more cars at a lower price. If your way is to sell less with a higher margin you may well stick at 1% all the time.
To keep around 10% share in later years I probably have to give up producing parts completely ... (rejecting researches and buying then from supplier)
A warning: Never have the same price for two ore more different cars in a branche. In this case the orders are bugged. Took me quite a long time to find out
Does anyone know if the three different controls for advertisement (newspaper, campaign, exhibitions) have any impact ?
Only thing I found out is to have the funding for advertisement at around least 5% of the sale region turnover, otherwise the sales go down.