Getting stuck in place tends to happen when people with Parkinson's disease are anxious or feel rushed, or when they're navigating small passageways like going in and out of elevators
or through a doorway. It's embarrassing and awkward for them.
One bit of advice is to resist physically or verbally tugging on your mother. Spouses do this all the time -- take the person's hand and pull her along. Or sometimes verbally or through eye contact, the spouse nudges the patient to "come along." None of these approaches are helpful or effective.
So never rush your mother, because the freezing will go away spontaneously. Sometimes, just sitting down for a minute (if she's in a place where that's possible) will relieve the problem. Another trick is to place your foot on the ground in front of her and tell her to carefully step over it -- this could break that freeze. You might want to have a doctor demonstrate this before trying it.
Freezing is a forward-motion problem, so your mother can also try rocking side to side. Or, counting one, two, three, four, and walking to that marching beat may work. You can count out loud along with her. Another good verbal trigger is to say, "Try lifting your knee up, Mom." And by not thinking about walking forward but just about lifting the knee, she may then be able to break the pattern.
It's best to talk with your mother ahead of time about the ways you might help when she freezes. What would she like you to do? Give a verbal cue like "knee" to remind her to raise her knee, or "march" if a regular beat is her best way to get going? This way, you have an agreed-upon system in place and she'll have some control over how you help.