Parallel gnu-make skips commands - gnu-make

Question

Given an empty file a.c and the following Makefile -
CC=gcc
l.a : l.a(a.o)
ranlib l.a
echo done
l.a(a.o) : a.c
clean::
rm -f l.a a.o
Running the command "make clean ; make" gives the following output -
rm -f l.a a.o
gcc -c -o a.o a.c
ar rv l.a a.o
a - a.o
ranlib l.a
echo done
done
Running the command "make clean ; make -j2" gives the following output -
rm -f l.a a.o
gcc -c -o a.o a.c
ar rv l.a a.o
a - a.o
Why does "make -j2" skip the commands listed under the first rule?
This occurs consistently on a 32-bit single processor system with gnu make 3.79.1
Does not occur on a 64-bit multi-core system with gnu make 3.81.
This is a very shortened example of a larger Makefile where I ran into the problem.
So, I made the example Makefile a bit more "complex" as follows -
AR := flock make.lock ar
RANLIB := flock make.lock ranlib
CC=gcc
l.a : l.a(a.o) l.a(b.o)
echo doing ranlib
$(RANLIB) l.a
echo done
l.a(a.o) : a.c
l.a(b.o) : b.c
clean::
rm -f l.a a.o b.o
The flock() trick is my invention (perhaps a re-invention) to serialize "ar".
Output -
> make clean ; make -j1
rm -f l.a a.o b.o
gcc -c -o a.o a.c
flock make.lock ar rv l.a a.o
ar: creating l.a
a - a.o
gcc -c -o b.o b.c
flock make.lock ar rv l.a b.o
a - b.o
echo doing ranlib
doing ranlib
flock make.lock ranlib l.a
echo done
done
rm a.o b.o
> make clean ; make -j2
rm -f l.a a.o b.o
gcc -c -o a.o a.c
gcc -c -o b.o b.c
flock make.lock ar rv l.a a.o
flock make.lock ar rv l.a b.o
ar: creating l.a
a - a.o
a - b.o
rm a.o b.o