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If the network is not transmitting well, if some frames or packets are dropped or lost, then pixelation is the usual result, or 'blockiness' (i.e. coarsening of the detail to the point of the pictures looking like a bunch of colored squares), etc.
The network's capacity is probably overloaded. (That is, too much bandwidth needed, but not enough throughput to meet the need.) Also, there might be a defective component in it, that either causes slowing or loss of packets. But lack of sufficient bandwidth is the likeliest cause.
If you're getting a picture at all even part of the time... then the network itself is set up well enough to transmit. I would say that network setup isn't the problem, if a picture can be seen or video can be seen (even if pixelated, blockiness-showing, etc.)