I have normally found failed tacks are more of an issue in light airs.
No disrespect intended but it is possible the problem stems from technique not the boat.
This may be teaching grandmother... but:

A technique that helps is to bear away briefly before turning into the tack. This can build a little more speed but crucially gives more yaw momentum to the turn before the wind is lost. This can be enough to carry you through the tack. Conversely if you simply bear up harder and harder slowly you will lose all way before you can make the tack and may well fall back to the same tack as the boat finds its own equilibrium again. On a dingy shifting the crew weight early can also help force through a tack quicker due to the change of shape of the wetted area but on a Mac it will make little difference. Backing the jib (holding it sheeted on the current tack until the wind fills it again) can help turn the boat quicker but at the cost of greater loss of way.
Too much rudder can be as much of a problem as being too slow and gentle. If the helm is turned too hard it can kill a lot of way from drag before it begins to impart momentum to the turn.

Macs are not noted for pointing all that close to wind (a consequence of the planning hull shape). Consequently they have to tack through a greater angle than most high performance modern yachts.
Say the boat is on a port tack, the hull is healing to starboard such that the shape of the wetted surface is centred to the right of the keel. As she comes into wind she will roll upright and at the same time pitch up due to the loss of forward forces from the sails, the addition of drag forces from them flogging and from the increased drag of the rudders. This results in a bow up motion putting the centre of buoyancy well behind the keel.
As the hull comes upright it moves the centre line of the wetted displacement inline with the keel. This nullifies the horizontal lift from the keel removing the horizontal force of the keel to port. The combination of the loss of this force with the shift in the effective buoyancy results in a yaw moment clockwise, i.e. to starboard which is against the tack.
This is what sends you straight back where you started so you need enough yaw momentum to overcome this before the boat comes upright and you need to keep enough way so that the keel will
keep producing some force to help.

One thought is the rotating mast on an M, does yours move freely? The leading edge and mast profile have a huge effect on the aerodynamics of the main and this could be a factor.

The best advice is just get out there and play, try turning the helm more and less, try bearing away first, try backing the jib. Then try the same on someone else's Mac and see if it is any better or worse.
The good part is you have the water all to yourself at this time of year!

Dave

Sorry John, I was assuming it might be you not the boat but as you know it's a new problem and if you haven't changed the rig I can only guess it is hydrodynamic and your dagger board and rudders must be the prime suspects.
Providing the hull is reasonably free of fouling the only other contributor would be the outboard adding drag if you have it down at the time.

Regards
Dave.

Ah, I see . There was talk out the AGM about getting a mold of a brand new dagger board that Simon Armitage was about to fit . He got it shipped from Estonia I think?! Look on ebay but maybe call Simon in case we can get a mold. A quick search found someone who could make a mold in Salisbury . I have a pristine M rudder that needs copying also.

Roly