Brief Answer:
It is due to generalised 
Atherosclerosis .
Detailed Answer:
Hello
Thanks for your query,based on the facts that you have posted it appears that you are facing problem of 
Erectile Dysfunction.
I shall be happy if you can post me few more details to help me in making comments on your problem.
1) Are you diabetic?
2) Do you have hypertension?
3) Do you have 
high cholesterol levels?
In view of you having a stroke few  years back your problem of erectile dysfunction is mostly due to generalised atherosclerosis involving penile artery which is main source of blood flow to the 
erectile tissue of penis.In such a situation the drugs like Viagra or 
Vardenafil are also not effective in achieving hard sustainable erection to enjoy sex.
I would advise you to consult qualified 
Urologist and get Colour Doppler Ultrasound Scanning  of the penis done to confirm the arterial insufficiency to penis (Reduced blood flow to erectile tissue) as a cause of your ED.
If confirmed it would be better to get 
penile implant surgery done to have enjoyable sexual  life and satisfy your partner.
Hope I have answered your query ,please feel free to ask if you have more questions ,
I shall be happy to help you 
Thanks and Regards.
Dr.Patil.