When your PCM detects a certain amount of slip rpm'* in lockup, it disengages and will not attempt lockup again on that 'run'. You can cycle it (reset) by doing this without stopping the car. Pay attention to the tach and give us a good description of what it does on FIRST lockup for that run cycle, as it may give an indication if it'* even trying. No SES light? It'* an indicator of nasty fluid, which can be an indicator of clutch wear (usually 2nd gear clutches on the 60'*).
Check your fluid level, smell, and color after driving as well, while it'* stirred up.