Space can be an issue. Android has a habit of stopping that when there's 1gb of space left, though it might vary with different phones. Try making some space.
Go through your app list in the settings and clear cache. Things like YouTube, Twitter, Facebook, browsers and games have a habit of taking up a couple hundred extra megabytes.
Images could've built up over time. Move them to an SD card, upload them to somewhere else like an image hosting site/Google Drive or delete them.