USB thumb drives are often problematic when connecting to an iPad. Many of these devices require more power than the iPad can directly provide - often necessitating connection via an intermediate self-powered USB hub.
Some USB devices, with lower power requirements, are reportedly usable if connected using Apple’s own Lightning to USB adapter - the latest version of which usefully allows simultaneous connection of power via a secondary Lightning connector (resolving power needs of many other devices).
You should also be aware of limitations associated with iOS and USB storage. Support for file types are highly limited; many file systems (such as NTFS) are not supported; file and folder structure and naming must typically conform to the structure typically found on a digitial camera card. The USB device cannot be formatted to a compatible file system by iOS; formatting to a compatible file system must be performed externally using a PC or Mac.
Other proprietary USB devices exist, together with an associated device App, to facilitate file transfer between the iOS and storage devices, however, support for file types generally remains limited to those natively supported by iOS.
I hope you find this to be of some help.