This study presents and interprets some conceptual difficulties junior-level physics students experience with Ampere's law. We present both quantitative data, based on students' written responses to conceptual questions, and qualitative data, based on interviews of students solving Ampere's law problems. We find that some students struggle to connect the current enclosed by an Amperian loop to the properties of the magnetic field while some students do not use information about the magnetic field to help them solve Ampere's law problems. In this paper, we show how these observations may be interpreted as evidence that some students do not see the integral in Ampere's law as representing a sum and that some students do not use accessible information about the magnetic field as they attempt to solve Ampere's law problems. This work extends previous studies into students' difficulties with Ampere's law and provides possible guidance for instruction. (Contains 6 figures.)