Loop over Publisher Combine framework

I have the following function to perform an URL request:

final class ServiceManagerImpl: ServiceManager, ObservableObject {
    private let session = URLSession.shared

    func performRequest<T>(_ request: T) -> AnyPublisher<String?, APIError> where T : Request {
        session.dataTaskPublisher(for: self.urlRequest(request))
            .tryMap { data, response in
                try self.validateResponse(response)
                return String(data: data, encoding: .utf8)
            }
            .mapError { error in
                return self.transformError(error)
            }
            .eraseToAnyPublisher()
    }
}

Having these 2 following functions, I can now call the desired requests from corresponded ViewModel:

final class AuditServiceImpl: AuditService {
    private let serviceManager: ServiceManager = ServiceManagerImpl()

    func emptyAction() -> AnyPublisher<String?, APIError> {
        let request = AuditRequest(act: "", nonce: String.randomNumberGenerator)
        return serviceManager.performRequest(request)
    }

    func burbleAction(offset: Int) -> AnyPublisher<String?, APIError> {
        let request = AuditRequest(act: "burble", nonce: String.randomNumberGenerator, offset: offset)
        return serviceManager.performRequest(request)
    }
}

final class AuditViewModel: ObservableObject {
    @Published var auditLog: String = ""
    private let auditService: AuditService = AuditServiceImpl()

    init() {
        let timer = Timer(timeInterval: 5, repeats: true) { _ in
            self.getBurbles()
        }
        RunLoop.main.add(timer, forMode: .common)
    }

    func getBurbles() {
        auditService.emptyAction()
            .flatMap { [unowned self] offset -> AnyPublisher<String?, APIError> in
                let currentOffset = Int(offset?.unwrapped ?? "") ?? 0
                return self.auditService.burbleAction(offset: currentOffset)
            }
            .receive(on: RunLoop.main)
            .sink(receiveCompletion: { [unowned self] completion in
                print(completion)
            }, receiveValue: { [weak self] burbles in
                self?.auditLog = burbles!
            })
            .store(in: &cancellableSet)
    }
}

Everything is fine when I use self.getBurbles() for the first time. However, for the next calls, print(completion) shows finished, and the code doesn't perform self?.auditLog = burbles!

I don't know how can I loop over the getBurbles() function and get the response at different intervals.

Edit

The whole process in a nutshell:

• I call getBurbles() from class initializer
• getBurbles() calls 2 nested functions: emptyAction() and burbleAction(offset: Int)
• Those 2 functions generate different requests and call performRequest<T>(_ request: T)
• Finally, I set the response into auditLog variable and show it on the SwiftUI layer