Conjugation occurs between cells of the same species too. For this to occur cell have to be close to each other. Now, if you have an isolated population of bacteria that never gets in contact with an F+ bacteria then this population would stay F-. Also not all conjugation events are successful, mechanical perturbations can disrupt the pilus through which genetic material is transferred.
This old article can give you good details:
THE ORIGIN OF BACTERIAL SPECIES - GENETIC RECOMBINATION AND FACTORS LIMITING
IT BETWEEN BACTERIAL POPULATIONS ARNOLD by W.RAVIN
F+ shares, however, an important property of bacteriophage: after infection
it behaves as though it were an addition to the bacterial genome. As an addition, it can furthermore exist in one of two forms: in one form, it
is an unintegrated part of the bacterial genome, and may be replicated either more or less rapidly than the "chromosomal" or linked genes (being
infectious or lost through dilution, respectively); in its other form, it
is integrated in the sense that its replication is coordinated with the
replication of the host's genes, as though attached to them. In this latter form, found in Hfr bacteria, the F+ factor is incapable of infecting other bacteria (while permitting conjugation with them) and also prevents
superinfection of its host bacterium by a nonintegrated F+ factor.
Taken from the linked paper. So you can see that are situations where conjugation occurs yet the F- won't become F+.
What is particularly interesting about the F+ factor is the fact that there
seem to be a number of different kinds. An F+ factor making a bacterium
fertile with some F- cultures does not necessarily make it capable of conjugating with others (49).
So not every F+ cell can conjugate with another F- cell.