my serry flute has been sounding rather dull lately, a few times it has sounded as if something is stuck in it. It is oiled, cleaned and played regularly, i dont understand were i’m going wrong! 
Is your Seery of delrin or wood?
–James
Is this one of Desi’s polymer flutes or wooden?
If polymer, I’d consider giving it a good bath in some warm water with dish detergent…couldn’t hurt. I’d also look at the joints - is the thread loose, do you need more, is it sealing correctly. Then, if all is good, I’d suspect the cork in the headjoint might be culprit. Is it sealing well? You can always replace it if it’s not.
Now, if it’s a wooden flute, does the dull sound exist throughout the scale or start a certain point down? You could have a crack somewhere, and cracks usually don’t effect the sound too much until you’re playing notes around or below the crack.
Also, on either wood or polymer, it’s good to look around the embouchure hole for anything that could be hanging there or stuck on the sides, and the same holds true for any of the tone holes.
Oh, and since I seem to be rambling on here, it’s not keyed is it? A leaky key can cause similar problems to a crack - notes below the key won’t sound as good.
For such a simple instrument, there sure are a lot of things that can effect the tone…
Eric
You might also want to check the cork stopper. Sometimes they can get a bit manky and affect tone.
Remember to measure the original position before you move it!
Harry
After three years of playing on my Seery, the cork became loose and I had the same problem in the sound than you.
Just drop the cork in boiling water for a minute (no more cause you won’t be able to stick it back to place!). And that’s it!
That’s a year ago, and no problem still then!
Good luck!
Leo
Another possibility would be a leaking tuning slide…it would cause the whole flute to be weak and unresponsive.
–James
Thanks for the advice people! I should have explained that my flute is a polymer. I have had a wee look at the tread and the joints and they seem fine. but i’m not sure what i should be doing with the cork, i dont want to ruin the flute 
. however after reading all the suggestions i do suspect that the cork is cause of all the bother, so what exictely should i do with it?
Why not call Desi Seery:
Desi Seery
40 Wolftone Square West
Bray, County Wicklow
Ireland
Tel: +353 (1) 2866192
Kevin Krell
That probably would be the best thing to do, but just thought i’d try and sort it myself, i dont want to bother the man if its something fairly simple.
Desi’s really nice, and quite the talker, if you call during normal work hours in Ireland I doubt you’d be bothering him.
As for the cork, use a dowel rod or chopstick to mark how far in from the back of the head (after removing the pastic cap) the back of the cork is, then you can use the dowel through the slide end to push the cork out. Do what Leonard suggests, put a little cork grease on it, and slide it back in from the backside until it’s in as deep as the mark you made on the dowel…it’s pretty simple really, but we were all nervous the first time we did it, and you often have to use the dowel ack and forth to get it back to the exact right place (not that a mm or two will make a big difference).
Eric
PS - Welcome back Kevin…how ya feeling?