You cannot clearly explain the source (s) of your funds
The majority of embassies will assume that applicants are seeking permanent entry into their country until you prove them otherwise. And one way to prove them otherwise is to present evidence that you have strong economic reasons to return to your country following your stay. What other way to do this than your bank statement? Hence, ensure that your cash inflow and outflow are credible. They do not want to give visas to criminals.
Your visa agent cannot answer basic visa questions
If your visa agent promises to deliver to you a visa without asking you basic questions that only you have answers to, you should feel very uncomfortable about that application. It may most likely be rejected.
You have a record visa denials
If you have a record of refusals from a particular embassy, they will know that you arguably are desperate to leave your country. If you have been refused once at any embassy, they have the record. Hence, do not submit any visa application without consulting reliable visa businesses for advice. The more you are rejected, the more your visa application will be denied and vice-versa.
You apply for the visa category
There are different visa categories and, for every visa you are applying for it must correspond with your lifestyle. If it does not support the category of visa you are applying for, that is a strong signal, which you address before even submitting your application or else you will suffer the embarrassment of a rejection.
You have an uneasy feeling about an answer
If you have answered a question on your application but you don't feel comfortable about the answer you have provided, then you need to take steps back and peruse your responses again. The reason is that, you will unconsciously communicate your uneasy feelings to your interviewer while answering that question; particularly if they seem interested in that particular answer.