Why wont he propose to me ?

My boyfriend and i have been together for a long time, we both love each other and have an amazingg relationship. We have spoken about getting married on many occastions but each time he says he wants to marry me although when i start getting into details he changes the subject. Latley, we he has been getting down on one knee..but then will do his shoelace up or something, each time my heart skips a beat.. but he just wont propose! I really need advice. Am i over reacting? is there something i can do to speed it up? drop hints?