Learn French: Le client est mécontent parce que le produit ne fonctionne pas. - The customer is dissatisfied because the product does not function.

Parce que is used to introduce a full clause with a subject and a conjugated verb (here, le produit ne fonctionne pas). You would use à cause de if you were only following it with a noun phrase, like à cause du produit (because of the product).

Is mécontent just content with a negative prefix?

Exactly! The prefix mé- often gives a word a negative or opposite meaning, similar to "dis-" or "mis-" in English. So content (happy or satisfied) becomes mécontent (unhappy or dissatisfied).

Could I use marche instead of fonctionne in this sentence?

Yes, you could say le produit ne marche pas. When talking about devices, machines, or systems, marcher and fonctionner both mean "to work." Fonctionner sounds slightly more formal, while marcher is very common in everyday spoken French.

The customer is feeling an emotion (dissatisfaction). Doesn't that mean we should use the subjunctive mood?

Not here! It's true that expressing emotions often triggers the subjunctive in clauses introduced by que (like Il est mécontent que...). However, parce que (because) is a conjunction of cause that states a fact. It always takes the normal indicative mood, even if the main sentence describes a feeling.