Lines do not reach the border of node

Question

I want to make a tikz-picture of a Hasse diagram and by problem is that the lines between nodes stop a bit short of the border of the node and I can't figure out how to fix this. Here is my code:

\begin{tikzpicture}[scale=\tikzscale,
source/.style={draw,thick,rounded corners,inner sep=2pt}]
\node[source] (min) at (0,0) {$\emptyset$};
\node[source] (A1) at (-2,1) {$a$};
\node[source] (A2) at (-1,1) {$b$};
\node[source] (A3) at (0,1) {$h$};
\node[source] (A4) at (1,1) {$c$};
\node[source] (A5) at (2,1) {$d$};
\node[source] (B1) at (-3.5,3) {$ab$};
\node[source] (B2) at (-2.5,3) {$ah$};
\node[source] (B3) at (-1.5,3) {$bh$};
\node[source] (B4) at (-0.5,3) {$ad$};
\node[source] (B5) at (0.5,3) {$bc$};
\node[source] (B6) at (1.5,3) {$ch$};
\node[source] (B7) at (2.5,3) {$dh$};
\node[source] (B8) at (3.5,3) {$cd$};
\node[source] (C1) at (-3,5) {$abh$};
\node[source] (C2) at (-1.5,5) {$adh$};
\node[source] (C3) at (0,5) {$abcd$};
\node[source] (C4) at (1.5,5) {$bch$};
\node[source] (C5) at (3,5) {$cdh$};
\node[source] (max) at (0,6.5) {$abcdh$};
\draw[thick] (min) -- (A1);
\draw[thick] (min) -- (A2);
\draw[thick] (min) -- (A3);
\draw[thick] (min) -- (A4);
\draw[thick] (min) -- (A5);
\draw[thick] (A1) -- (B1);
\draw[thick] (A1) -- (B2);
\draw[thick] (A1) -- (B4);
\draw[thick] (A2) -- (B1);
\draw[thick] (A2) -- (B3);
\draw[thick] (A2) -- (B4);
\draw[thick] (A3) -- (B2);
\draw[thick] (A3) -- (B3);
\draw[thick] (A3) -- (B6);
\draw[thick] (A3) -- (B7);
\draw[thick] (A4) -- (B5);
\draw[thick] (A4) -- (B6);
\draw[thick] (A4) -- (B8);
\draw[thick] (A5) -- (B4);
\draw[thick] (A5) -- (B7);
\draw[thick] (A5) -- (B8);
\draw[thick] (B1) -- (C1);
\draw[thick] (B1) -- (C3);
\draw[thick] (B2) -- (C1);
\draw[thick] (B2) -- (C2);
\draw[thick] (B3) -- (C1);
\draw[thick] (B3) -- (C4);
\draw[thick] (B4) -- (C2);
\draw[thick] (B4) -- (C3);
\draw[thick] (B5) -- (C3);
\draw[thick] (B5) -- (C4);
\draw[thick] (B6) -- (C4);
\draw[thick] (B6) -- (C5);
\draw[thick] (B7) -- (C2);
\draw[thick] (B7) -- (C5);
\draw[thick] (B8) -- (C3);
\draw[thick] (B8) -- (C5);
\draw[thick] (C1) -- (max);
\draw[thick] (C2) -- (max);
\draw[thick] (C3) -- (max);
\draw[thick] (C4) -- (max);
\draw[thick] (C5) -- (max);
\end{tikzpicture}

Can anybody help me? Thanks!