If it's got a wooden block, it may have dried out and shrunk in the few weeks you've not been using it, making the windway more open than it should be. In which case it will get better if you keep trying - it will rehumidify.

Good instruments use cedar for the block, which doesn't expand and contract with humidity. It has a distinctive taste, like chewing the end of a high-quality pencil. If it doesn't taste like that, your whistle might have a block made of unsuitable wood. Not a lot you can do about that unless you fancy spending hours doing micro-scale sculpture to replace it. The tolerances involved are a few thousandths of an inch.

With a wooden recorder or whistle, you can pop the block out by putting as thick a dowel as possible up the bore of the instrument and slamming it on the floor (tip: put a sock over the end or you'll be searching all over the room for the block when it flies off). This means you can clean the block and windway properly - if you use the instrument a lot, the wetted surfaces will have accumulated condensed saliva, dried food and fungal/bacterial gunge. If I remember the the Clarke's construction right, the block is usually retained by a couple of crimped indentations on the sides of the block, and I'd expect the dowel-slam technique to just pop them open - I don't have a Clarke here to try. If the block is held in with pins, you need to get them out somehow.

You can't usually pop the block with a plastic recorder, though they're easy enough to clean with detergent. The transparent Yamahas (like the one I'm playing in the photos on my webpage) are nice fom this viewpoint because you can see the gunge building up sooner.