Blocking effect after reconstructuring the image. Jpeg compression

The image is splitted into 8x8 blocks each of which are processed through DCT, quantization, huffman encoding/decoding, dequantization and inverse dct and used to recontruct the image. But there is a horrible blocking effect after this compression process. What would be the reason why this is happening? I simply create a new matrix for storing the data as big as the original image. And each block after processed is copied into new image. For example first 8x8 block is put from (0,0) to (7,7). The second block is put on the indices between (0,8) and (7,15)