Incomplete arrows and missing arrow heads in TikZ

Question

I have the following plot which looks good, but there are two issues:

1. Not all arrows have arrow-heads. I have no idea why this is.
2. Some edges are not long enough. For example, the left edge of the root does not touch the root at the base.

What can I do to fix this?

\tikzstyle{every node}=[align=center, fill=white, rectangle, draw, rounded corners]
    \tikzset{external/remake next}
    \begin{tikzpicture}[->, thick,scale=0.8, level distance=3em, growth parent anchor=south]
    \node {Cryptographic\\primitives}
    [sibling distance=25em]
    child {
        [sibling distance=10em]
        node {Public-key\\primitives}
        child{ 
            [sibling distance=.5em]
            node {Public-key\\ciphers}
        }
        child{ 
            [sibling distance=.5em]
            node {Signatures}
        }
    }   
    child {
        [sibling distance=10em]
        node {Symmetric-key\\primitives}
        child{ 
            [sibling distance=10em]
            node {Symmetric-key\\ciphers}
            child {
                node {Block\\ciphers}
                child {
                    node {Lightweight\\block ciphers}
                }   
            }
            child {
                node {Stream\\ciphers}
            }
        }
        child{ 
            [sibling distance=2em]
            node {Hash\\functions}
        }
        child{ 
            [sibling distance=2em]
            node {Pseudo-random\\sequences}
        }
    }
    ;
    \end{tikzpicture}

Answer 1

Problems were due to the fact that you specified manually [sibling distance=..] at each step covering the arrows. It is better to declare once the distance by means of level 1/.style={...}.

Here is my MWE:

\documentclass{scrartcl}

\usepackage{tikz}
\usetikzlibrary{trees,arrows}

\begin{document}
\tikzset{every node/.style={align=center, fill=white, rectangle, draw, rounded corners}}
\tikzset{level 1/.style={sibling distance =25em}}
\tikzset{level 2/.style={sibling distance =10em}}
\tikzset{level 3/.style={sibling distance =7em}}
\tikzset{level 4/.style={sibling distance =5em}}

    \begin{tikzpicture}[-stealth,thick,scale=0.8, level distance=3em, growth parent anchor=south]
    \node {Cryptographic\\primitives}
    child {
        node {Public-key\\primitives}
        child{ 
            node {Public-key\\ciphers}
        }
        child{ 
            node {Signatures}
        }
    }   
    child {
        node {Symmetric-key\\primitives}
        child{ 
            node {Symmetric-key\\ciphers}
            child {
                node {Block\\ciphers}
                child {
                    node {Lightweight\\block ciphers}
                }   
            }
            child {
                node {Stream\\ciphers}
            }
        }
        child{ 
            node {Hash\\functions}
        }
        child{ 
            node {Pseudo-random\\sequences}
        }
    };
    \end{tikzpicture}

\end{document}

Graphical result:

To let all arrows meet correctly boxes, one can remove the option growth parent anchor=south and increasing the level distance. For example:

\begin{tikzpicture}[-stealth,thick,scale=0.8, level distance=6em]

This will give: