Why would someone be unable to take a deep breaths after cardiac ablation?

May not be related: While possible, it's very rare that an ablation leaves a patient with any lasting symptoms afterwards. The microscopic area of heart tissue destroyed is generally far too small to be felt. In very rare cases, it's possible the procedure could initiate inflammation in the space lining the outside of the heart, which might cause pain on deep breathing or breathlessness. Again, this is very rare.

Pericarditis: When we do this, particularly if it's an atrial fibrillation ablation, there can be irritation of the lining of the heart (the pericardium) and it causes a sharp pain with deep breaths, worse when laying down, better when leaning forward. There are also more serious causes though they are more rare by far. Speak with your electrophysiologist if you have concerns. Simple meds may help.

Ablation: this is quite unusual, have you doc check it but most likely the discomfort is coming from chest wall not heart.