Depending on what server-side language /platform youre using, I find i always have error logging turned off and am not seeing any of that, so that is where i would start. And depending whats available in your dev environment, you would want to make sure that you're getting 200 status code from the eloqua link. Even if it doesnt return any error message saying what exactly is wrong, eloqua does return status. The last thing, it may help to look at an eloqua generated form (i'm sure you've done this) there are a few fields that are hidden and required for submission, i think they are form name and site id. and the typical last thing, would be to remove all the validation from the form asset in eloqua because if there is anything funky coming through, it might be refusing it, and you're not seeing an error message.
if you're feeling adventurous, you can maybe try the REST API, but honestly probably more complicated:
Best of luck!