[GNC] could not obtain lock message when opening GnuCash

[David Long]

Hi,

I am using GNucash on 3 machines, who share the file on Dropbox. I make
sure only one version is open at any one time. When opening GnuCash, I get
the message * GnuCash could not obtain the lock to( folder\filename). That
database may be in use by another user, in which case you should not open
it."  I always ignore the message and open it anyway, as the backups give
me the same message. I am worried that this might give me problems at some
stage. What should I do?
thanks
David