问题的原因可能是由于某些设置不正确或某些步骤缺失导致的。下面是一种可能的解决方法，其中包含了使用AVAssetWriter将图像写入MP4文件的代码示例：

import AVFoundation

func writeImagesToMP4(images: [UIImage], outputFileURL: URL) {
    // 创建写入视频的输出URL
    guard let videoWriter = try? AVAssetWriter(outputURL: outputFileURL, fileType: .mp4) else {
        print("无法创建AVAssetWriter")
        return
    }

    // 定义视频的宽度和高度
    let videoWidth = Int32(images[0].size.width)
    let videoHeight = Int32(images[0].size.height)

    // 配置视频输入
    let videoOutputSettings: [String: Any] = [
        AVVideoCodecKey: AVVideoCodecType.h264,
        AVVideoWidthKey: videoWidth,
        AVVideoHeightKey: videoHeight
    ]

    let videoInput = AVAssetWriterInput(mediaType: .video, outputSettings: videoOutputSettings)
    videoInput.expectsMediaDataInRealTime = true

    // 配置视频写入器
    if videoWriter.canAdd(videoInput) {
        videoWriter.add(videoInput)
    } else {
        print("无法添加视频输入到AVAssetWriter")
        return
    }

    // 开始写入会话
    videoWriter.startWriting()
    videoWriter.startSession(atSourceTime: .zero)

    // 创建一个调度组，用于等待所有视频帧都写入文件
    let writingGroup = DispatchGroup()

    // 获取写入队列
    let writingQueue = DispatchQueue(label: "com.example.videoWritingQueue")

    // 写入视频帧
    var frameNumber = 0
    for image in images {
        writingGroup.enter()

        // 将当前帧写入文件
        writingQueue.async {
            if videoWriter.status != .writing {
                writingGroup.leave()
                return
            }

            if let pixelBufferPool = videoInput.pixelBufferPool {
                var pixelBuffer: CVPixelBuffer?
                let status = CVPixelBufferPoolCreatePixelBuffer(nil, pixelBufferPool, &pixelBuffer)

                if let pixelBuffer = pixelBuffer, status == kCVReturnSuccess {
                    CVPixelBufferLockBaseAddress(pixelBuffer, [])
                    let data = CVPixelBufferGetBaseAddress(pixelBuffer)

                    let colorSpace = CGColorSpaceCreateDeviceRGB()
                    let context = CGContext(data: data, width: videoWidth, height: videoHeight, bitsPerComponent: 8, bytesPerRow: CVPixelBufferGetBytesPerRow(pixelBuffer), space: colorSpace, bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue | CGBitmapInfo.byteOrder32Little.rawValue)

                    if let context = context {
                        context.clear(CGRect(x: 0, y: 0, width: Int(videoWidth), height: Int(videoHeight)))
                        context.draw(image.cgImage!, in: CGRect(x: 0, y: 0, width: CGFloat(videoWidth), height: CGFloat(videoHeight)))

                        CVPixelBufferUnlockBaseAddress(pixelBuffer, [])

                        let presentationTime = CMTimeMake(value: Int64(frameNumber), timescale: 30)
                        videoInput.append(pixelBuffer, withPresentationTime: presentationTime)

                        frameNumber += 1
                    }
                }
            }

            writingGroup.leave()
        }
    }

    // 等待所有视频帧写入完成
    writingGroup.wait()

    // 完成写入
    videoInput.markAsFinished()
    videoWriter.finishWriting {
        if videoWriter.status == .completed {
            print("视频写入完成")
        } else if videoWriter.status == .failed {
            print("视频写入失败: \(videoWriter.error?.localizedDescription ?? "")")
        } else if videoWriter.status == .cancelled {
            print("视频写入被取消")
        }
    }
}

使用此方法时，您需要提供一个UIImage数组和要输出的MP4文件的URL。该代码将按顺序将每个UIImage帧写入MP4文件，并在完成时输出相关的状态消息。确保在调用此方法之前，您已经将所有图像加载到内存中。

希望这可以帮助您解决问题！