I just got dentures over implants. My lower will not stay snapped in. What can I do to help it to stay?

First, how long ago did you have the implant denture made. I am not sure what "just got" really means. If it has been over 3-6 months, then the plastic pieces or o-rings that snap to the implant abutment could be fatigued and need replacing. Have they never snapped in at all or has this changed to now not snapping after a period of time when they did? There could be food or some other debris blocking the pieces from attaching properly. These plastic attachments or o-rings usually need to be replaced with fresh, new, elastic ones every few months.  If you have new plastic attachments, then the implants and abutments need to be carefully evaluated for any problems.
If your implant retained denture is pretty new, you should make an appointment with the dentist who made it and go over your concerns so they may be addressed.

Many times the denture needs to be relined to fit the tissue better and the o-rings need to be replaced.Sometimes its best to add additional implants to help stabilize the denture and achieve a more secure fit.

First, you must make sure there is no food or debris in the attachment that is screwed in to your implant.  You can clean this with a toothbrush, toothpick, or waterpik(oral irrigator).  The snaps will not engage if they are blocked from seating by food debris.  If this does not solve the problem, then you need to visit your dentist.  The snaps may have bent and if this is the case, they will need to be replaced.  

I have done hundreds of snap in dentures and one common cause is that the denture needs fitting to tissue.  Many dentures are tissue supported and implant retained.  Snap the denture in and if displaces when you push down in different areas this is likely the cause.Bite issues can also displace dentures so the bite should be crefully checked.Food in the inplant or denture can prevent proper seating.Sometimes imlant placement are unfavorable and additional implants are needed.