Hai, Welcome to HCM.
You have not mentioned about your age.
Anyhow you have to certain facts about our sexual habits.
One's sexual erections will be instant,strong, frequent and lasting long during the age of 16-23 years.
After that you will not have such an erection in your life spontaneously, only with a lady or with drugs.
Likewise you will have frequent erections and an urge for sex within hours during your first few months of sex contact with your wife or other girls. It will slowly wean and you will get erections only after an interval of day, then days, then week, after weeks etc.
That is you got stimulated easily and frequently with first few months of your married life and then it needs more time to get stimulated, then only you will get erections. That is because initially you are fresher then you are accustomed. You will get stimulated with different girls faster than with your wife with whom you are used to. This common to almost everybody.
Ageing also another factor to get your hormones at a low tide. A sixty year man cannot be compared to a sixteen.
Anyhow this lower frequency of sex with your wife will not affect your fertility
. Sex on alternate days or thrice in a week is sufficient for the sperm to meet ovum.
Don't get depressed. If your wife is not getting pregnant within another 6 months, You both meet your physician and get investigated.
Get your baby very soon.