objects deleting with out me deleting them

[Kahn]

, i am making a map with a medi center, in there, there are cylenders taht move up and down and can heal u, i made the cylender hollow. I will bsp the map, test it out and it works how i want it to
The next day i will open the map to work on it again, the cylender is not there, and when u bsp then, i am geting Entry error(mirrored planes or somtgihn) and there at the coords of 0 0 0 but i cant see them. I the only time it does this is in this room. i ahev tryed so many time to do this but it always does this, i have a bigger cylender else where in the map but it doesnt do it.

How can i fix this? i am geting made having to remake this every time. PLZ HELP

[Nkenobi]

sometimes a get a wierd problem were an object duplicates itself, but I don't know it, adn there are to objects exacly the same overlapping each other, if this is the case, it will be invisable.

[Kahn]

i ahve done that but they never turn invisble, i can click where they should be and it is still not there

First check filtering isn't making them invisible by going to View->Filter.
Then run Brush Cleanup first (blue and green arrow pointing in a circle in 1.4.0).