Overdenture with two locators: why are the attachments not engaging into the Locators?

Maybe, you used too much pressure during the impression phase. If you press too much you have to press too hard to clip the two parts of the locators and it make them uneasy to engage correctly

Make sure when securing your metal housings with acrylic that you are using the black laboratory processing male retainer and not the colored ones. ( be sure to use the white tissue guard when doing this). Also make sure you are using the non-parallel engaging male retainers without a center piece. These are for stand alone implants when female portions are not placed on a cast bar. The non-parallel allow for a 10-20 divergence of your implant placement. This one factor could be your problem if you have not already done this. You should at a minimum be getting one of the attachment to hold in any regard.

correction: I meant to say when female portions are placed on a cast bar.

Grind the old metal housings out of the denture base. Place 2 new metal processing caps back on the implants and use some silicone (fit checker) to make the denture fully seats around the implants without the housings touching the recessed holes drilled out in the denture. If there is any show thru, enlarge the holes in the denture. Recheck. Now that it fits passively, use the white block out rings over the locators and use the product by Voco called Quick UP. Best stuff out there. You can time the set so you know when to pull the dentures out. Any air bubbles you can add more material, light cure and polish. Works like a charm. Then change the black processing nylon insert to what ever retention you feel you need. Usually pink or white. If the implants aren't parallel, use the extended range nylon inserts. That should do it. Simple. One other thing. Make sure you using Locators and order from Zest the male caps. There are knock-offs out there and there is a slight size difference.

Good advice so far. It is also possible that the denture extensions do not permit passive seating. Consider having the patient wear the denture without the attachments or housings for at least a day before proceeding as recommended above; adjust the denture as necessary to prevent displacement. On another note, do we call extended range inserts "female" and standard Locators "male" given the nature of the coupling?

I no longer ask the lab to pick these up for me. They were consistently unable to pick them up accurately. Confirm that the housings are tall enough. After the housing is in place with the black housings, confirm that no part of the abutment is visible above the gingiva (I have had one lock on), block out with rope wax if needed. Pick up all housings at the same time rather than one at a time. Use a pickup material that bonds to the denture acrylic (cold cure is fine, it is just slow), otherwise you increase the risk of the denture cracking or teeth falling out. I hold the denture in with finger pressure, if the patient occludes too hard it may bottom out the black processing inserts which eliminates the resiliency and may prevent them from seating properly. I have had poor results with the extended range inserts. I know that others have had better luck. In non-parallel circumstances I use 15 or 30 degree GPS abutments. As the previous poster noted, you cannot use Locator inserts with GPS housings. However, a Locator insert in a Locator housing will attach fine to a GPS abutment. A Zest rep told me not to exceed 12 lbs of retention with the inserts, otherwise most patients will be unable to insert them completely (4 pink or 2 white). A quick check of the Zest website shows that all nylon inserts are referred to as "male" (regular or extended range).

The lab used the wrong stone. I send a confirmation jig with all my implant cases. If you don't get a jig don't have the lab process the caps.

Denture extensions is the problem. Must have passive seating of the appliance prior to picking up the housings. I personally use ERA pickup. QuickUp is another similar option.

I neglected to mention and didn't see it in other posts, be sure to drill vent holes in the denture when picking up the attachments to prevent hydraulic pressure. Failure to do so may prevent the denture from seating properly. BTW, in case it wasnt obviousI was being facetious about the male/female attachment description.