make sometimes fails

[Robin Atwood]

I am playing around with KDevelop 3.9.94 and sometimes a build works OK and 
sometimes I get:

...build> make myclass.o
make: *** "no rule to make target myclass.o". Stop.

Any idea what causes this? If I do the make manually, the build is OK. 
(Incidentally, you can't select/copy the build output.)

TIA
-Robin
-- 
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Robin Atwood.

"Ship me somewheres east of Suez, where the best is like the worst,
 Where there ain't no Ten Commandments an' a man can raise a thirst"
         from "Mandalay" by Rudyard Kipling
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