Paste comes in chunky and creamy, like peanut butter.
Stencil can come out good using larger, coarser solder balls, than syringe.
If the stuff comes out of a little jar, it might be too coarse for a syringe. Then when you push on the syringe, the emulsion can separate, some excess liquid getting squeezed out, initially. And then blobs of solder without enough flux, intermittently fall out.
Temperature is critical, too.
The guys who do it like this this swear by specific brands. Blueskull and some others have mentioned GC10. So, basically, you can't try one solder paste and expect it to work worth a darn.