I am trying to fix the words under the lines in the headings (for some reason they are stacked next to each other). Furthermore, I dont know why the numbers between brackets do not show in the tables.
Here is the LaTeX code
\documentclass[11pt,a4paper]{article}
\usepackage{amssymb, amsmath, bm}
\usepackage[flushleft]{threeparttable}
\usepackage{float, booktabs, makecell, caption, tabularx}
\usepackage{siunitx}
\begin{document}
\begin{table}[htb!]
\caption{\textbf{Estimates for the TVTPs and average TVTP in each regime}}
\label{table:mstvtp_estimation_results}
\centering\scriptsize
\renewcommand{\TPTminimum}{\linewidth}
\setlength{\tabcolsep}{2pt}
\setlength{\extrarowheight}{2pt}
\begin{tabular}{@{} l *{6}{S[table-format=1.3]} @{}}
\bottomrule
\specialrule{0.4pt}{\aboverulesep}{0pt}
& \multicolumn{2}{c}{MS-TVTP}
& \multicolumn{2}{c@{}}{MS-DRA-TVTP} \\
\cmidrule(l){2-3} \cmidrule(l){4-5}
& \thead[l]{$S_t = 1$} & \thead[l]{$S_t = 2$} & \thead[l]{$S_t = 1$} & \thead[l]{$S_t = 2$} \\
\Xhline{0.5pt}
Constant & $ \bm{7.000} $ & $ \bm{-0.999} $ & \bm{6.223} & \bm{2.101} \\
& (0.112) & (0.460) & (0.134) & (0.512) \\
GDP Growth & $ \bm{1.834} $ & $ \bm{-0.626} $ & \bm{1.147} & \bm{-0.028} \\
& (0.659) & (0.277) & (0.647) & (0.291) \\
FFR & 0.420 & 0.369 & -0.323 & -0.493 \\
& (0.602) & (0.514) & (0.748) & (0.544) \\
CPI Growth & -0.292 & $ \bm{2.000} $ & -0.327 & \bm{1.141} \\
& (0.839) & (0.597) & (0.760) & (0.612) \\
Avg. p_{11}& 0.928 & & 0.922 \\
Avg. p_{22}& 0.993 & & 0.934 \\
\Xhline{0.8pt}
\end{tabular}
\medskip
\begin{tablenotes}[flushleft]\footnotesize\smallskip
\item Note: This table reports estimates of the MS-TVTP model. Panel (A) presents estimates for the autoregressive coefficient matrix $\boldsymbol{F}$, vector of means during periods of normal interest rates ($\boldsymbol{\mu}_1$) and low interest rates ($\boldsymbol{\mu}_2$), decay parameter $\lambda$ and average transition probabilities $p_{11}$ and $p_{22}$. Panel (B) presents estimates of the logistic function which is used to compute the time-varying transition probabilities. Bold entries denote parameter estimates significant at the five percent level. Standard errors appear in parentheses.
\end{tablenotes}
\end{table}
\end{document}
Thank you in advance.

siunitx's columns. – leandriis Jul 23 '19 at 17:32\begin{tabular}{@{} l *{6}{S[table-format=1.3]} @{}}into\begin{tabular}{@{} l *{4}{S[table-format=1.3]} @{}}? – Matthew Jul 23 '19 at 17:35