On the subject of maintenance surprises the newest soundings has a very funny piece about a guys fight with his head. A head water intake would block even though he:
1. Rebuilt the head twice
2. Checked the hoses, they would draw from a bucket no problem and were not collapsing.
3. Checked the seacock it was working properly.
4. Dived and put a wire in from the outside and the thru hull was not blocked.
In every case it would work for a couple of flushes then block. Sometime he would try it later and it would be fine then block again.
Try to figure it out.
No fair people who have read the article.