how to find out why my 2024 pension1099-R box 2a (taxable amount) that I entered in Turbotax is much smaller than line 5b (taxable amount)?

[NCperson]

do you have a second 1099-R?  Any 1099-R form WITHOUT the check mark next to Box 7 should sum to Line 5b.  Any 1099-R form WITH a check mark next to Box 7 should sum to Line 4b. 

I suspect you have multiple 1099-R forms.  Or you entered a 1099-R phantom form that should be not entered,