Learn Italian: Dubito che abbiano ignorato il reclamo, ma pretendo una risposta. - I doubt that they ignored the complaint, but I demand an answer.

The main verb dubito expresses uncertainty in the present, which triggers the subjunctive in the following clause. Because the action of ignoring happened before the doubting, we use the past subjunctive to show that the event is already in the past. We form it using the present subjunctive of the helper verb avere (abbiano) plus the past participle ignorato.

Does pretendo mean I pretend?

No, pretendere is a classic false friend! It actually means to demand or to strongly expect something. If you want to say to pretend in Italian, you would use the verb fingere.

Why isn't there a subject pronoun like loro before abbiano ignorato?

In Italian, subject pronouns are usually dropped because the verb ending tells you who is doing the action. The verb form abbiano is specifically the they form of the subjunctive helper verb, so it is already clear who we are talking about.

Since pretendo expresses a demand, shouldn't it trigger the subjunctive too?

It would trigger the subjunctive if it introduced a new clause with a new subject, like Pretendo che rispondano (I demand that they answer). However, in this sentence, pretendo is just followed by a direct object noun (una risposta), so there is no new action to put into the subjunctive.