In the morning I’ve learned that your body is usually very stiff compared to the rest of the day, so it’s normal not to be able to do much during that time. I would still suggest doing it in the morning because you’ll have less chances of getting distracted but if you can in the afternoon then I’m all for doing what’s best for your motivation. A calendar is also a great way to keep track and set goals. You can try and write down each small goal for each day of the month (for ex. May 22 “hold stretches for longer” or “try a new pose”) or you can just cross off the days that you did do yoga and look back each month or week and see how well you kept up your streak and reflect on what you might need to do different in order to keep a better streak. Sometimes it’s “I need to do only 10 mins a day because one hour has proven to be discouraging” or “ I should set up my mat and have a video or plan ready for the next day so I'm not discouraged to find it everytime I need to do yoga”. What ever works best your you is always the better option when it comes to reaching your goal. Just remember it's okay to mess up it just shows you're trying.
If you want, you could try waking up and hour or two earlier and start warming up your body, so you have more time to do your morning yoga. And if you're tired, maybe sleep after you've finished.