Re: what might be the reason if a job fails in acqusition phase and application phase
For mload, you can read the this small chapter "Handling Teradata MultiLoad Errors". It explains about Acquisition Error Table and Application Error Table. Reading the error and log tables will give a vivid explanation of the root causes.
Similarly, you can read fastload document on this chapter: "Handling Teradata FastLoad Errors"
where it highlights about constraint violations,conversion errors, duplicates ...
Reading the document and real time experience will give the best proper explanation.