Also if you are continuously monitoring their SpO2 don't be surprised if you see it dip a little while suctioning, but this should rebound pretty quickly. In addition to checking your vent settings, check all connections including power strips and alarm cords & check the top of the circuit & heat & moisture exchange filters. If they've coughed up any gunk hanging around in the circuit &/or the HME is saturated their pressure & breath rate will go up.
Circuits attached to a concha can also get full of moisture, make sure you are emptying the collection chamber & be careful with repositioning the pt & tubing so that you don't accidentally send the moisture back toward their trach. Keep tubes of sterile saline nearby in case the secretions are thick so you can lovage. If they are bleeding with sections cold saline lavages can work wonders but you may need a physicians order for the cold saline technique.
Don't be afraid to ask other nurses & the respiratory therapists when questions come up... Good luck