One more possibility is that the input Base64 stream has line breaks, which have to be omitted. And just read correct length would not be enough, it needs to remove all line breaks, truncate to the closest divisible by 4 length, and using the left tail with the next iteration. 

Periodically I see, that some systems may replace symbols such as "+" or "/" in Base64 with something like URL compatible.

Updated

Class CodeGolf.NatoTranslator
{

ClassMethod ToNato(i As %String) As %String
{
 s l="lfa,ravo,harlie,elta,cho,oxtrot,olf,otel,ndia,uliett,ilo,ima,ike,ovember,scar,apa,uebec,omeo,ierra,ango,niform,ictor,hiskey,ray,ankee,ulu"
 f j=1:1:$l(i){s k=$e(i,j),o=$g(o)_" "_$s(",.!?"[k:k,1:$zcvt(k,"U")_$p(l,",",$a(k)#32))} q $zstrip(o,"<=W")
}

ClassMethod Test(val = "If, you can read?") As %Status
{
  set res = ..ToNato(val)
  zw res
  #dim methodObj As %Dictionary.MethodDefinition
  set methodObj = ##class(%Dictionary.MethodDefinition).IDKEYOpen($ClassName(), "ToNato")
  Write !,"Size: ", methodObj.Implementation.Size
}

}

Size: 255