Easy to test the taps.

Remove the fuel line, take the filler cap off and it should pour out very quickly...

If not, you either have a faulty or blocked tap, a blocked filter (see Mezo's photo) or a vacuum lock..

You can remove the taps to inspect them. Check the filter isn't gummed up.

You can blow them out using compressed air.

It's pretty simple to work out once removed.

If they DO flow freely off the bike, ensure you have no kinks or bends in your fuel line. Weird angles can make a HUGE difference as can in-line filters.