A few things I've heard that might be relevant, it might be erroneous but it makes sense to me.
Back in the Themata days the value of land equal to 1 pound of gold came with the requirement to contribute a cataphract, but by Basil this had increased to land to the value of 4 pounds.
During a long period the farming style in Anatolia changed from mixed farms with many middle and lower class owners to more and more large livestock ranches owned by the Dynatoi. This depopulated Anatolia to a considerable extent and gave the Turks the perfect environment for their lifestyle when they arrived.
Themata are good for defence but Tagamata are good for offense, so a lack of usefulness on the offensive rather than blind neglect was a major reason for the decline of the Themata.
Dunno how all that fits in, but I think they do somewhere.
Well that was the point of the Themata. They were designed to keep raiding in check and provide a large pool of manpower for the army. They were good at this from everything that I've seen and while they were not great on the offensive, they were good enough to provide support the Tagmata when more troops were needed for an offensive.
During the height of the Theme system the ideal situation in case of an invasion that the local theme could not defeat on its own was for the local thematic soldiers to try and harass the enemy and keep them in place, limit their progress as much as possible until the Tagmata arrived along with reinforcements from other themes to drive the invasion off.
As far as I know the weakening of the Themes pre-Manzikert was intentional on the part of the Doukas emperors who sought to limit the power of the Strategoi and instead place smaller tagmatic forces, usually mercenaries, under Dux who owed their loyalty to the Emperor himself, at least in the short term. This is understandable given the problems the Byzantines had with civil wars, but it was incredibly short-sighted.
If I remember correctly, the Emperor of the time even went so far as to completely disband the Armenian theme and abandon it to the migrating Turks, losing not only a vital buffer area between the raiders and the economic heartland of the Empire, but also a valuable area for recruitment (rough mountainous areas tend to be a good place to recruit soldiers) and about
40,000 experienced troops. Manzikert, I'd think, could potentially have been avoided almost entirely if not for this Emperor, Constantine X I believe, and his astounding degree of complete incompetence.