Quadmix dose worked only once

[mybppp]

On the advice of my doctor, I started with 30 units (0.3 cc) of Quadmix but with no results. Then he gave me 40 units (0.4 cc) of Quadmix to be injected at home. I managed to inject it at home and got an erection lasting two hours. I was very happy and the doctor gave me 2cc of Quadmix in a bottle so that I can inject later on when required.
However, on all subsequent tries whenever I injected I didn't get erection. The doctor revised the dose and advised to go for 0.5 cc, 0.6 cc and 0.8 cc but still I am not getting erection.... I told the doctor that I feel I am not injecting properly but he replied that its not possible because its very hard to inject it wrongly and that I am tensed thatswhy the drug is not working.
Has this happened with anyone else that a dose which worked once only and then even the double dose is not giving any results? Is the doctor true about my anxiety level or there could be a problem with my injection technique (I've read somewhere on the net that if the dose previously worked is not working now than probably you are too deep or too shallow).
Someone on these forums have suggested to check if the plunger depresses easily, it is an indication that you are at right position. In my case the plunger depresses really easily. Also someone commented that plunger should be pulled a bit backwards and if some blood comes in the syringe, it is an indication of the right position but I rarely see any blood on pulling back the plunger (even though it pushes forward easily!)
I am living in this part of the world where it is very difficult to find specialist doctors, even the injection medication is scarcely available. So will be grateful to have comments of you all.