I presume you are looking at a map like this, which doesn't give a lot of context. Here's a little quick export from a database I am working on/with of US nuclear tests for Frenchman Flat:

Date	Operation and Shot	Yield est.	Type
1951-01-27	Ranger Able	1	A
1951-01-28	Ranger Baker	8	A
1951-02-01	Ranger Easy	1	A
1951-02-02	Ranger Baker-2	8	A
1951-02-06	Ranger Fox	22	A
1952-04-01	Tumbler Able	1	A
1953-05-08	Upshot-Knothole Encore	27	A
1953-05-25	Upshot-Knothole Grable	15	A
1955-04-15	Teapot MET (Military Effects Test)	22	A
1955-11-01	Project 56 Project 56 No. 1	0	A
1955-11-03	Project 56 Project 56 No. 2	0	A
1955-11-05	Project 56 Project 56 No. 3	0	A
1956-01-18	Project 56 Project 56 No. 4	0	A
1957-06-24	Plumbbob Priscilla	37	A
1958-10-15	Hardtack II Hamilton	0	A
1958-10-22	Hardtack II Wrangell	0	A
1958-10-26	Hardtack II Sanford	5	A
1962-07-14	Sunbeam Small Boy	1	A
1965-02-18	Whetstone Wishbone	5	U
1965-05-14	Whetstone Cambric	1	U
1965-06-16	Whetstone Diluted Waters	4	U
1966-04-25	Flintlock Pin Stripe	5	U
1966-09-12	Latchkey Derringer	8	U
1966-12-13	Latchkey New Point	7	U
1968-03-25	Crosstie Milk Shake	10	U
1968-08-27	Bowline Diana Moon	9	U
1969-09-12	Mandrel Minute Steak	5	U
1971-11-24	Grommet Diagonal Line	4	U

A = Atmospheric, U = Underground.

Some of these are pretty well-documented (like operation Ranger, and especially shot Grable, the famous "atomic cannon" shot). Some are obscure but still documented, like Project 56, which was a series of no-yield safety tests. And the underground tests are as under-documented as much of those tend to be.

Here is Area 16 (Shoshone Mountain):

Date	Operation and Shot	Yield est.	Type
1962-06-28	Nougat Marshmallow	10	U
1965-04-21	Whetstone Gum Drop	20	U
1966-06-15	Flintlock Double Play	10	U
1968-11-20	Bowline Ming Vase	16	U
1970-05-12	Mandrel Diamond Dust	10	U
1971-07-01	Grommet Diamond Mine	10	U

Which is also similarly as obscure as underground tests tend to be. Those "10" yields are just my (current) internal coding for yields the DOE lists as "low - under 20 kt" which is not that meaningful here. But if you look up any of those individual shots you'll find some documentation, e.g. Nougat Marshmallow: "Simulated high altitude effects shot in low pressure chamber, successful, x-ray effects of re-entry vehicles investigated." Neat.

For Area 18 (Buckboard Mesa), you have:

Date	Operation and Shot	Yield est.	Type
1962-03-05	Nougat Danny Boy	0	U
1962-07-07	Sunbeam Little Feller II	0.022	A
1962-07-11	Sunbeam Johnnie Boy	1	U
1962-07-17	Sunbeam Little Feller I	0.018	A
1964-12-18	Whetstone Sulky	0	U

Which is more of sort of the same. Operation Sunbeam / Dominic II are some low-yield nuclear tests (including of the "Davy Crockett" bomb). Nougat Danny Boy was also an "atomic demolition munition," very low yield. Whetstone Sulky was a Plowshare shot.

Anyway. No real mysteries here on the whole; I wouldn't call them "missing." They are just in somewhat different locations than the rest. In some cases it is clear why (e.g., some of the underground ones required specific kinds of rock, or for Operations Ranger and Sunbeam they wanted to have some troops running around and that required a different landscape/layout and that probably dictated the choice of location). I'm not sure looking at these three sites in particular gets one much by itself, though. If I'm missing the point of your question, let me know! :-)