This problem usually is because at some point the communication with iTunes or the Adobe Distribution Service has stopped and consequently the process stays on hold
In many of the cases are resolved by reloading the page. Clicking on the icon in the top right corner of the screen (a gear circle) will resume the process.
If the problem persists , you have to ' kill ' the application: When switching from one application to another, or simply by closing it. Actually, it won't stay closed but suspended, with all the options active. To really force the app to close, must follow instructions provided under this link:
If in any case a magazine remains blocked completely, or partially downloaded , it shall be restored from the library: Select the magazine, press the edit button and remove it from the shelf, then download it normally.
Please, provide iPad-model and IOS version in your queries.