Hi Harry and welcome! We're glad you're here.
Let me toss out a few ideas and you can let me know if any of them might make sense ...
Are you a coffee drinker?
I'm a first-thing-in-the-morning exerciser and my coffee is what gets me up and keeps me going!
Time of day makes a difference - we all have natural body rhythms and times when we feel more energetic (and times we just want to take a nap!) If your schedule permits, take advantage of your peak energy time of the day for your workout.
Are you eating good carbs? Are you eating before you exercise? What you eat (even if it's healthy) and when you eat can affect performance.
Are you taking a rest day during the week and not overtraining?
Just some thoughts ... I'm sure you'll get lots of other ideas.