The solder paste has dried up. Can you add a little soldering flux?
There are many kinds of diluents, using special solvent to solder paste, flux with diluent is best not to use, and, if it may produce flux, solder ball, collapse and other adverse consequences, please think twice!
Let the solder supplier send you some thinner
It's better not to add. It adds a lot to the reliability of the welding
