There appears to be a bug in Android’s Download Manager on Android 9 Pie that causes it to periodically crash and refuse to download files. New downloads will show up as queued, but will fail to download. I’m assuming that at some stage Google will release an update that fixes this, but in the meantime users can force close the Download Manager process, which will temporarily resolve the issue. Go to Apps & notifications, click the option to See all apps, tap the options menu in the upper-right corner, select Show system, select Download Manager from the list, then tap on Force stop. Note that it may also be necessary to clear Download Manager’s cache or storage in some cases.