Some backends may take longer than others to respond or complete, and some may decide to disable themselves if they are not suitable for the current buffer or encounter some unavoidable problem. A disabled backend is not tried again for future checks of the current buffer.
show the backends currently used and those which are disabled.
flymake-mode off and on again, or invoking
flymake-start with a prefix argument is one way to reset the
disabled backend list, so that they will be tried again in the next check.
Flymake also uses a simple logging facility for indicating important points in the control flow. The logging facility sends logging messages to the *Flymake log* buffer. The information logged can be used for resolving various problems related to Flymake. For convenience, a shortcut to this buffer can be found in Flymake's menu, accessible from the top menu bar or just left of the status indicator.
Logging output is controlled by the Emacs