By default, Starry Night reads in only 600 comets from the Comets.txt file. In Preferences ---> Startup ---> this can be changed to a higher number.
Linear 252P comes in as number 733 in the Comets.txt file. So, increase your Startup number to 734 and you should be able to see it in the Starry Night universal search:
LINEAR 17.5 0.0 0.673656 0.996104 190.9819 343.2902 10.4047 2457462.7834 2457386.5 10.0 252P MPC 95735
Note, you must restart the app after changing the startup number to make the changes take effect.
I tried this suggestion and three times it failed, even when I increased the comet data file to 740. Frustrating. Easy to find on Stellarium and that's free!
What Starry Night product and version number are you using?
Linear 252P is now listed as the 744th comet in the database. So, just to stay on top of this for the foreseeable future please increase the default readout significantly, i.e. 1500. Or, more if the comet ever fails to display.