Custom fluid can't flow

Currently I am working on a custom dimension, I made a new type of fluid called coaltar, which pulls entities to its source. I defined a custom feature which coaltar would randomly appears on the ground, and flow around to catch any possible entities.

But after entering the new dimension, the coaltar that appears on the ground just keep still ,until I put some blocks near them.

Then I switched the coaltar to water, I found that if I choose "water", the waterblock just keep still as the coaltar. But if I choose "water_still", the waterblock would flow automatically.

But for the custom fluid "coaltar", I can't find a "coaltar_still" when choosing block.

Then is there any way to solve the problem? Thanks for the help.