How long has it been in that pot?

Yes, along jeans lines pot too small and soil needs refreshed, and in addition orange trees from seed from the grocery (opposed to heirloom from friend/family garden) likely don't come true and you don't know what you are going to get. And MG likely not the...um...miracle in any case and you want to look at the formulations in citrus food and adjust your fert from there.

Dan

It could be one or a combination of many things, root bound, soil consistency, possibly over watered. As far as soil, if it's anywhere close to the right consistency and drains well, it should be fine for up to 2 years. Obviously people have different opinions about MG but using it at a lower strength more often is better than full strength in a container. I'd almost be willing to bet it's a soil issue. It's probably compacted and not allowing proper drainage or oxygen to the roots. What kind of soil was used?