I'd agree with most of the above, but I'll a note or two.
As said, pine can be rather demanding stuff to plane, or at least it can be demanding to plane to a flat surface with a uniformly good finish. The wavy grain is not too much of a problem, but the frequent knots are - they tend to have wildly reversing grain around them, they tend to be very hard compared to the clean timber, and they can have short-grain bits that break out in small, but deep, chunks. (Sometimes, it's easier to just accept that and later fill the holes with stopping or similar, especially if the part is intended for a paint finish.)
One approach, especially useful when there's a lot to come off, is to set the plane for a deep cut, and work ACROSS the grain. Be careful about break-out on the far side of the board; either stop short, or plane a bevel on the edge to take the breakout. You'll still have problems around the knots, and reducing the depth of cut to sort out particularly obdurate hard ones can help. Work the timber down almost to depth on either side, then go back and reduce the knot to level.
Having got the board to dimension, sharpen up the plane iron, set the cap-iron very close to the cutting edge, reset the plane for a very fine cut, and clean up the ragged finish on the surface working ALONG the grain. Try to work with the grain as much as you can, and if you meet the odd patch of reverse grain, turn the plane round and pull it towards you. Skew the plane across the knots; that can help a bit in getting a cleaner finish, though the sharp edge and fine cut will help far more.