Singer Concerto 3 - won't pick up the bobbin thread?

Since sewing very heavy material, we have replaced the needle (was slightly bent) Despite our best efforts st following the indtruction manual, we cannot get it to pick up the bobbin thread - suspect it has become mis-timed.
Any explicit instructions on resetting this would be very welcome. This has a horizontal bobbin and nylon gears (which look undamaged and mesh fine),cannot see any scribe lines/timing marks on the needle holder.
Thanks