I wonder if it because the solar credit is a "non-refundable credit". That means that the credit can not take Line 22 below zero.
Look at your 2023 and 2024 tax return. What is on Line 22 in each year? Let's start there.
The solar credit can reduce the tax you owe but can't increase your refund if you already owe no tax. You might owe little to no tax if you are middle income and have a few children under age 16, or you are living off investments rather than taxable wages, or are self-employed but don't show a profit on your tax return.
The unused credit can be carried forward indefinitely until you have a tax situation where you can use them. Just don't lose track and make sure to include the credit carry-forward on every tax return until you can use it. (Even though H.R. 1 (the "big beautiful bill") eliminates the solar credit starting in 2027, any credit earned in the past will still carry forward.)