mixing down/merging...

[redheadedboy]

Whenever I mix down (mono) two or more recorded parts (on the same track), the resulting mix down is always quieter than the original parts.

For example: I have a guitar track, but I want to append an extra bit to the end. This new part overlaps the previous part. To avoid the 'overlapping parts' dialog and to keep the screen uncluttered (yes, I'm anal, I know) I want to merge these tracks.

Problem is, the resulting mix is always quieter than the original parts. How do I keep the same volume when mixing down? What am I doing wrong?

Thanks in advance,
tim