Water and other clear liqids are mostly transparent. If exactly perpendicular enough photons might hit the surface of the liquid and bounce back. Even the best glass is only 94% transmissive, so you always have a change.
But one has to be careful. If the bottom of the container is not particularly reflective, it works. But on cannot measure water in a stainless steel container. Just too many photons hit the bottom and reflect.
So either your liquid is particularly transmissive to 940nm light, or the bottom of your container is particularly reflective.
If you container is clear - try putting a piece of black paper under it. It might help.
Otherwise you are going to have to look at the chemical properties of your liquid.
(Water vapor absorbs 940nm light, It's why we chose that frequency. There is less 940nm light at the surface of the earth, as a lot of that frequency got absorbed by the upper atmosphere.)
Most common liquids are largely water. It's why milk and coffee are no problem, but your's might be different.
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