As has been said already, Tadalafil, Sildenafil &c all require stimulation for them to work. Unfortunately things like stress, anxiety and guilt can all contribute to its effects, or lack of.
I take Tadalafil, mainly because it lasts up to 36 hours but that doesn't mean I have a hard on for 36 hours. 10mg works great for sex with the OH (or a wank even!), but when punting I usually take 20mg and sometimes erection doesn't last or I only get a semi. It is usually better with WGs that I have seen before and there is much less stress involved. However the guilt is always there!
Before punting, which is always early or mid morning for me, I take 10mg as I go to bed, then 10mg the following morning, two to three hours before my appointment. Also side effects with Tadalafil are much less for me. Little or no facial flushing. Took Sildenafil once at 100mg and had a head like a beetroot!