Of course there are hundreds of mutants that either lack flagella or flagellar motility is so poor that cells do not swim out from division sacks. Wild type cells, however, can also sit on the bottom of liquid cultures and/or grow only as clumps. The following conditions favor the presence of flagella, if the cells are capable of swimming. Most importantly, soon after cells reach stationary phase they will often lose their flagella. For best swimming use cells during exponential growth phase. Media of the lowest possible tonicity (for example Sager and Granick medium (M) as opposed to TAP or HSA), will favor flagellation. You can even put cells for several hours in distilled water to favor flagellar assembly. After a few hours they will lose their flagella. Always take cells from a fresh plate into liquid media, rather than passing from liquid to liquid. This observation is odd because cells lose flagella rapidly on solid media. In fact the first flagellar regeneration experiments (by Ralph Lewin) involved growth after agar to liquid transfer as opposed to flagellar amputation. If you really, really need your cells to have flagella consider treating your cultures with autolysin to free the cells from walls within which they divided.