You didn't provide a MWE, but I assume that you are using something like
\begin{enumerate}[label=\textbf{\arabic*}]
\item first
\item second
\item third
\begin{enumerate}[label*=.\arabic*]
which does make the second level enumerate items bold. This is not surprising, as you are forcing the label to be bold using label=\textbf{\arabic*}.
Instead, you should specify the font face separately, using
\begin{enumerate}[label=\arabic*,font=\bfseries]
\item first
\item second
\item third
\begin{enumerate}[label*=.\arabic*]
....
Here's a complete MWE to play with.
% arara: pdflatex
\documentclass{report}
\usepackage{enumitem}
\begin{document}
\begin{enumerate}[label=\arabic*,font=\bfseries]
\item first
\item second
\item third
\begin{enumerate}[label*=.\arabic*]
\item one
\item two
\item three
\end{enumerate}
\end{enumerate}
\end{document}