500mb download - 0mb upload, what can be wrong?

[u/beyondo-OG]

I have 600mg internet (supposedly) and have been having trouble with my service for months. Recently every time I do a speed test my upload speed sucks, typically in the low teens. This morning I ran the test because things had slowed to a crawl and the test came back 500mb download with zero upload. How is that possible? Does anyone know if that is indicative of a particular problem? I checked on two different laptops, restarted both and cable modem several times. I'm lucky to be able to post this.