Check all your CAN connections. Do a gentle pull test on each wire individually at each and every joint. If you are using connectors, ensure that every contact is seated properly in the housing. We had a big panic just before Bag & Tag because one of the CAN wires at the PDP had a contact was not seated properly in its housing.
I do not recommend soldering the wires together. It makes it more difficult to re-use your motor controllers in subsequent years. Secondly, soldering is a manual skill. Some people catch on quickly. Some will require a lot of practice to get acceptable results. There is not enough soldering needed in an FRC robot for even those who catch on quickly to become really proficient at it.