Cfrgtkky

My codes are follows:

documentclass{book}

usepackage{amsmath}

usepackage[silent,nohug,heads=littlevee,midshaft]{diagrams}



begin{document}





begin{diagram}[height=0.7cm]

H^{n}(k,(n)) &rTo& H^{n}(k,Q(n)) &rTo& H^{n}(k,D(n))

&rTo& 0qquadqquad \ 

dTo^{} && dTo^{cong} && dTo &&  \

H_eta^{n}(k,(n))&rTo&H_eta^{n}(k,Q(n))&rTo&H_eta^{n}(k,D(n))

&rTo^{text{onto}}& H_eta^{n+1}(k,(n)).

end{diagram}



end{document}

And the output came as:

Both the marked arrow head position went wrong, is it possible to fix it? advise.

I can't find diagrams package in my TeX distribution, but you can get the same result with tikz-cd:

documentclass{book}

usepackage{amsmath}

usepackage{tikz-cd}

usetikzlibrary{arrows.meta}

tikzcdset{arrow style=tikz, diagrams={>={Straight Barb[length=5pt,width=5pt]}}}

begin{document}

[ 

begin{tikzcd}[cells={anchor=west}]

H^{n}(k,(n)) ar{r}ar{d} & H^{n}(k,Q(n)) ar{r}ar{d}[swap, xshift=-2pt]{cong} & H^{n}(k,D(n)) ar{r}ar{d}

&[10pt] 0 \[10pt] 

H_eta^{n}(k,(n)) ar{r} & H_eta^{n}(k,Q(n)) ar{r} & H_eta^{n}(k,D(n)) ar{r}[yshift=2pt]{text{onto}}

& H_eta^{n+1}(k,(n))

end{tikzcd}

]

end{document}

The littlevee option is very buggy; the tips are decently placed only in vertical arrows, see also https://tex.stackexchange.com/a/308352/4427

There's not much more you can do than removing the option head=littlevee.

Debugging diagrams.sty is essentially impossible, because the code is obfuscated. As far as I can see, the arrows are boxed during package loading. That the littlevee arrows are inherently broken can be seen also in the manual itself (page 11)