Why my variable is always getting 0 as value after looping in a subscription

Question

I want dropVolume get the value to use it in another method after calling my

getAllDropsVolumePerDate(date) method. but every time I get dropVolume = 0

this is my function :

 dropVolume = 0;
  getAllDropsVolumePerDate(date: string) {
    this.campaignService.getAllCampaigns().subscribe((res) => {
      res.forEach((campaign) => {
        campaign.drops.forEach((drop) => {
          if (drop.dropDate === date) {
            this.dropVolume = this.dropVolume +drop.dropVolume;
          }
        });
      });
     return this.dropVolume;
    });
    return this.dropVolume;
  }

this is my getAllCampaigns() in the Campaign service :

 getAllCampaigns() {
    const campaigns = collection(this.firestoreDB, 'mail_campaign');
    return from(
      collectionData(campaigns, { idField: 'id' }) as Observable<Campaign[]>
    );
  }

Please any help ?