Try trouble shooting the following: Here’s some ideas
Are the rig and mesh aligned on the axis properly?
Have you applied scale and rotation on everything you’re weighting?
Did you make sure to keep the x – axis mirror option on when rigging? The weight painting gets wonky when things are less symmetrical.
Have you removed double vertices, checked your mesh for bits that don’t aren’t connected, and recalculated normals?
Is your mesh or armature too large? Sometimes scaling it down can help.