TLV2772: Why does my summing amplifier not work?

Dear Sir/Madam,

I build a summing circuit for my piezoelectric microphones with TLV2772, which is shown below.

However, it does not work.

The output voltage after summing is smaller than that before summing (Vout is smaller than V1 and V2).

I also tested the summing part with function generator (V1 and V2 were provided by function generator), but it did not work (Vout is a little bit higher than V1 or V2, but much smaller than V1+V2) .

Why does this happen? How can I solve the problem?

Please help. Thank you very much.

Best, Hai