Different BIMIX dosage at night than in the morning?

[AverageHusband&Dad]

My wife and I usually do the deed Saturday and Sunday mornings when the kids are still asleep and I usually use 5 units of bimix (30/1) which gives me a good 25 minutes (Cant get too crazy, the kids are in the next rooms). A couple times when we have a "Date Night" and the kids are at our parents for the night, I inject 5 units around 9:00 or 10:00 pm and it's not as full as when I use 5 units in the morning (around 10:00 am). The first evening time it happened I snuck off after 30 minutes and injected just 2 additional units on the other side and that seemed to do the trick giving me a good 40 minutes or so. Also were always pretty intoxicated by then. Just wondering if this happens to anyone else. Only been using injections about 3 months now so this is still new to me. I haven't asked my Urologist who prescribed it, figured I'd ask here first.