Hey, it happens when you get older. You just can't train hard every session. Wait, did you think I was talking about something else. I'm not that old.
Seriously though, I have been sticking with the same 5th Set bench program for three months now. At first, I was completing my two bench days a week with my main lower body lifts as well. I would deadlift on Mondays after my high rep bench session and I would squat after my heavy bench Fridays.
I cannot remember the life circumstance, but I ended up moving my lower body work to what were essentially assistance days. So I started deadlifting on Wednesdays finishing with assistance and squat on Saturdays and finish with assistance then also.
Before too long, I was feeling run down and beat up. However, nothing really changed, so I thought. Same conditioning and training. But it wasn't, training was different. Originally, I really loaded up two days a week, and then the other two were assistance days where I just got work done.
I recently switched back to the two hard days a week and two assistance days a week and this was during my heaviest training. Guess what happened, I immediately started feeling better. What did I learn, it's not that I couldn't handle the exercises or the volume, I couldn't recover from four days of intense training versus two.
As you get older or if you are just having difficulty recovering don't necessarily take anything out of your training if it hasn't been an issue. Perhaps juggling things around to decrease your number of "intense" training days might be the ticket.
