Fitness improvements don't always happen in a predictable way. Sometimes we're just not as strong or energetic for no apparent reason, or we hit a plateau. If you get to a point where you're struggling to complete the day's challenge, you can try one of the following:
Take a couple of days off and start again where you left off
Do the challenge on alternate days rather than every day
Instead of increasing the effort every day, stay at the same level for 2 or 3 days and then go onto the next day of the challenge
Obviously these solutions mean the challenge will last longer, but you'll still benefit from doing it.