Maybe you entered it in the wrong place? Did it show up on another line like line 4 or 5? Social Security should be on line 6a and the taxable amount on 6b. Or maybe none of it is taxable on 6b?
Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0