[Nathan Streeter]

It's really hard to lower the CG of a full-weight, already complete robot...

If your CG isn't centered front-to-back, move weight (the battery, perhaps?) around to accommodate.

As has been suggested, if you have any weight to spare, add it as low as you can... I prefer adding "functional weight" if possible... (i.e. beef up the bottom of the frame, rather than just strapping on dead weight)

Move any heavy objects lower, where possible... the battery, an electronics board, that gearbox/motor for your elevator, etc. Consider using two lighter motors up top on your shooter... Most people are using BB550s, BB775s, or FPs for the shooter wheels - two CIMs is more power than you need.

Take off any weight you can above ~8"... Reduce fasteners, consider rivets instead of bolts, tap rather than using a nut if you can, replace heavy upper frame members with lighter ones. A lot of upper-frames are over-engineered... 80-20 or 1x1x.125" is .5 pounds per foot, twice as much as 1x1x.0625" box. PVC rollers can also be heavy... consider removing excess ones, going to smaller diameters, or at least speed-holing them in acceptable areas. Also, if you have belting for your harvester, powering only one side, putting a thin sheet of lexan on the other side.

Add weight to your bumpers, if their center is below 8" (which it has to be this year). You have 20 pounds for bumpers... consider using steel angle instead of aluminum, using more fasteners, or some such thing.

Consider shortening your entire upper assembly. Given the high-trajectory arcs, you could likely shoot from lower (~40") without worrying about your shots being blocked.

If you really can't lower it sufficiently, and can't use a retractable wheelie bar to help you, consider removing a motor/actuator from up top entirely. It's better to have limited functionality then to spend half of every match on your side!