Filament won't feed on its own - does my extruder need cleaning?

I'm building an Ormerod 2 and attempting my first print. I've had a lot of issues with this thing, but until recently gripping filament wasn't one of them! I'm trying to print with PLA, and a few weeks ago I had no issues at all feeding filament into the extruder. Now that same filament doesn't go through on its own -I have to push it really hard into the extruder to get it to feed into the Bowden tube at all. It simply won't grab the filament on its own - if I'm not standing there pushing it into the extruder with force, it fails to feed the filament on its own.

What do I need to do to troubleshoot this problem? I haven't actually managed to print anything yet, so I'm not sure if this is a normal issue or something that I've built incorrectly.

It is very likely that your hotend is the culprit, not the extruder. With PLA it is very common that you get heat creap, which means that the heat travels to far upward in the hotend, where it softens the PLA to a point where it builds a barrier. If you have a full metal hotend this poses a bigger problem since soft PLA sticks to steel very good. A PTFE inlined hotend is more forgiving in this respect, but it can still go wrong.

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If the extruder can't move properly rule out the options first.
Remove the tube and extruder for a good cleanout - PLA love to leave dust and pieces behind in the most unusual places.
Then check if the grub screw on the extruder is tight - these little bastards are often overlooked....
Clean the hobbed bolt or whatever transports your filament including the bearing on the other side.
Put the exturder together and check if the filament moves through.
Set the pressure so the filament is transported safely without too much deformation.
Add the tube connector and make sure nothing can get caught there, add the tube and let the extruder run till the filament is through.
If all that comes out fine you should be good to go again unless your hotend/nozzle plays up too.