When is a VOR shown on maps?
See the Interagency Air Cartographic Committee (IACC) Specifications:


  [IACC 2] 3.9.3.8 Compass Roses
  
  Compass roses, as illustrated in Appendix 6, shall be shown centered on all VHF NAVAIDs, when space allows.
  
  Preference shall be given to those serving enroute functions or located in areas not served by other VHF NAVAIDs.
  
  [IACC 5] 3.7.4.5 Compasses Roses
  
  Compass roses shall be charted centered on those NAVAIDs which have routes predicated on them. Compass roses may be shown around other NAVAIDs where placement of the compass rose does not create additional unnecessary clutter or congestion. If compass roses overlap and congestion hinders readability, the compass rose of the NAVAID with the most congestion may be deleted. Compass roses shall be shown oriented to slave magnetic North. Cardinal direction values shall not be shown. Compass roses shall be .75" in diameter and in very congested areas may be reduced in size to .5" in diameter.


In groundschool I was taught that this is actually based on VOR service volumes (which otherwise aren't explicitly marked at all on VFR sectional charts): the compass rose is not shown around VORs with Terminal service volumes. This might be a general rule, but it is not entirely true however. You provide a perfect example: OJC and RIS are both Terminal VORs.

The key differences: 1) OJC has the V10-12 airway, and 2) map is more congested around RIS.