Problem with F&T tests in newey west

[parastou amirifar]

Hi statalist.I work with time series data and my thesis is about the effect of a health intervention on the efficiency of hospitals. Based on the articles I read, the regression model I chose is right. I ran the following codes but F & T test were not significant. What do you think about the problem?
[numdate month mdate = dateid, pattern(YM)][list, noobs clean]

crossid dateid effici~y mdate
1 201103 1 2011m3
1 201104 .89083 2011m4
1 201105 .67241 2011m5
1 201106 .96226 2011m6
1 201107 .93791 2011m7
1 201108 .70067 2011m8
1 201109 .87219 2011m9
1 201110 .58491 2011m10
1 201111 .83638 2011m11
1 201112 .66353 2011m12
1 201201 1 2012m1
1 201202 .95982 2012m2
1 201203 1 2012m3
1 201204 1 2012m4
1 201205 1 2012m5
1 201206 1 2012m6
1 201207 .68553 2012m7
1 201208 .96768 2012m8
1 201209 .83967 2012m9
1 201210 .75885 2012m10
1 201211 1 2012m11
1 201212 1 2012m12
1 201301 .68645 2013m1
1 201302 1 2013m2
1 201303 .67857 2013m3
1 201304 1 2013m4
1 201305 1 2013m5
1 201306 .84861 2013m6
1 201307 .9611 2013m7
1 201308 .91349 2013m8
1 201309 .89032 2013m9
1 201310 .66038 2013m10
1 201311 .62244 2013m11
1 201312 1 2013m12
1 201401 1 2014m1
1 201402 .99205 2014m2
1 201403 .87226 2014m3
1 201404 .7449 2014m4
1 201405 1 2014m5
1 201406 .74214 2014m6
1 201407 .98742 2014m7
1 201408 1 2014m8
1 201409 .96351 2014m9
1 201410 .92451 2014m10
1 201411 .69182 2014m11
1 201412 .83036 2014m12
1 201501 .9626 2015m1
1 201502 .74138 2015m2



[di ym(2013, 02)]
637

[tsset crossid mdate, monthly]
panel variable: crossid (strongly balanced)
time variable: mdate, 2011m3 to 2015m2
delta: 1 month

[itsa efficiency, sing treat(1) trp(637) posttrend]
panel variable: crossid (strongly balanced)
time variable: mdate, 2011m3 to 2015m2
delta: 1 month


Many thanks.