  Recent spectroscopic measurements of the equivalent widths of the resonant Be
II doublet and Ca II K lines and their ratios in expanding nova ejecta indicate
surprisingly high abundances of $^7$Be with a typical mass fraction
$X_\mathrm{obs}(^7\mathrm{Be}) = 10^{-4}$. This is an order of magnitude larger
than theoretically predicted values of $X_\mathrm{theor}(^7\mathrm{Be})\sim
10^{-5}$ for novae. We use an analytical solution of the $^7$Be production
equations to demonstrate that $X_\mathrm{theor}(^7\mathrm{Be})$ is proportional
to the $^4$He mass fraction $Y$ in the nova accreted envelope and then we
perform computations of 1D hydrostatic evolution of the $1.15\,M_\odot$ CO nova
model that confirm our conclusion based on the analytical solution. Our
assumption of enhanced $^4$He abundances helps to reduce, although not
completely eliminate, the discrepancy between $X_\mathrm{obs}(^7\mathrm{Be})$
and $X_\mathrm{theor}(^7\mathrm{Be})$. It is supported by UV, optical and IR
spectroscopy data that reveal unusually high values of $Y$ in nova ejecta. We
also show that a significantly increased abundance of $^3$He in nova accreted
envelopes does not lead to higher values of $X_\mathrm{theor}(^7\mathrm{Be})$
because this assumption affects the evolution of nova models resulting in a
decrease of both their peak temperatures and accreted masses and, as a
consequence, in a reduced production of $^7$Be.
