Usually when you ignite an engine on orbit a short kick of ullage trusters is enough. It is because once the fuel settled enough to start the engine the trust of the engine will keep the fuel settled. This is not true in the fuel transfer situation, there a single kick is not enough. If you are using ullage motors and not internal bladders, you have to keep producing the micro-trust for the whole transfer duration. Be mindfull that pumping a rocket-full of fuel won’t be an instantenous process.

You need acceleration to settle the fuel and to keep it there. Once the acceleration stops, the fuel begins to unsettle.