How can I fix my lower implant to stay snapped in place? (Photo)

There are various reasons for loose dentures. Easiest would be to change the nylon cap to a more retentive type. Usually locator comes in 3 different types. Blue, Pink and White. White will have the maximum retention.


Next ask your prosthodontist to check for any overextension or under extension in your dentures.
Third ask your doctor to check the way upper and lower dentures meet. If upper and lower dentures are not in equilibrium with each other likely that problem will recur. 

How long ago was this lower denture made?When were the locator insert pieces last replaced?Depending on the answers you could need new plastic inserts all the way up to a reline and occlusal equilibration.  

The attachments can be changed to be more retentive. You should speak with your dentist about changing out the pieces in the denture to hold on to the implants better.

Changing the plastic attachment within the denture could help, but often the abutment itself may be worn and may need to be replaced as well.
The real problem though is quite likely to be the occlusion (the way the bite comes together). It will need to be adjusted in a way that when you chew and move your jaw left to right, the dentures by way of having a balanced occlusion, can be more retentive.
If occlusion is the real problem, then there will always be accelerated wear of the components, and even if either the plastic insert or the abutment itself get changed, the issue is likely to recur.