The reason it is sometimes hard to fill a tank the first time is because they are tested with high pressure air (or nitrogen) for leaks. That air is often left in the tank and wont escape if you just open the valve. With that much air in the tank, there's no room for PP.
A good PP retailer will know to remove the brass coupler from his filler hose to open the safety valve in your tanks filler which will allow the air to escape. It can be a couple of hundred pounds of pressure and take several minutes to escape out the small filler orifice.