we found it isn't at the quality standard required to move forward

I tried to submit the work, but the response was “… we found it isn’t at the quality standard required to move forward …”

What is wrong with my design?
Why was it not accepted?

Thank you in advance

I suspect your icons were rejected due to variety. What I mean is that many icons differ from each other, e.g. in the thickness of the lines, which gives the illusion that they are from different sets.

I think you need to focus on standardizing the line thickness of each icon. Perhaps this is the main reason for rejection.

As well as the consistency and weight of these, the scale is very different between then with some much bigger than others, and there are a few which do not look centralised or positioned correctly.

Generally the quality looks a bit pixelated and it would be very easy to find similar freebie versions