1. 35% - Payment history: Whether you've paid past credit accounts on time
2. 30% - Amounts owed: The amount of credit and loans you are using
3. 15% - Length of credit history: How long you've had credit
4. 10% - New credit: Frequency of credit inquires and new account openings
5. 10% - Credit mix: The mix of your credit, retail accounts, installment loans, finance company accounts and mortgage loans

because you have no debt you would get low scores in 1,3 and 5 and high scores in 2 and 4

so you see having no debt can result in a low credit score.

Why do FICO® Scores fluctuate/change?
There are many reasons why a score may change. FICO® Scores are calculated each time they are requested, taking into consideration the information that is in your credit file from a consumer reporting agency at that time. So, as the information in your credit file at that CRA changes, FICO® Scores can also change. Review your key score factors, which explain what factors from your credit report most affected a score. Comparing key score factors from the two different time periods can help identify causes for a change in a FICO® Score. Keep in mind that certain events such as late payments or bankruptcy can lower FICO® Scores quickly

Now Experian claims to have "Credit Boost" which they say can increase your credit score merely by paying things like utility and phone bills on time. . that's their claim. I have not used it.