I can only speak for myself and quote the others who have posted here. What you are experiencing is the same as most posters to this board. While a minority have a natural firm erection within days or weeks, Most say that it takes months, even years to attain a natural erection. By natural, I mean one without the use of oral or injected medicine.
My urologist said not to worry until at least a year has passed without any progress. If you continue to use oral medication and try injections in the future, you will probably begin to have some success.
My progress has been slow, but with the help of injections, it is much better. Keep asking your doctor and the people here and let us know how things are going with you.