The burner manifolds in the oven each use a glow bar or electrode type igniter with a gas safety valve to ignite. Both oven manifolds are controlled by the Electronic Oven Control (EOC) board, which is your clock/timer on the control panel. If the surface igniters work and the broiler still works, the problem is not a gas related issue or a control board issue. More than likely, you have a weak or failed oven igniter. Igniters grow weak with age and occasionally need to be replaced. Just because an igniter glows, does not mean it is good. If you are experiencing longer ignition times, or no ignition, this is most likely the case. The following link explains a little theory about these igniters and how to replace one:
http://www.fixya.com/support/r3575727-replacing_an_oven_igniter_on_a_gas_rangeReplacement parts (if required) can be purchased at any of the following websites:
searspartsdirect.com
pcappliancerepair.com
appliancepartspros.com
repairclinic.com
Prices vary between these sites, so shop and compare. The first three sites listed also offer helpful exploded view diagrams to help you locate and properly identify the parts you need.
If you have any questions please post back with a complete MODEL NUMBER and let me know. The model number should be listed around the oven door or lower drawer opening on a Kenmore. I hope this helps you.