I can think of several possibilities.
The thyroxine tablets are effectively less potent than the ones you were taking. This might occur because they are a different brand, they are old, or they have been stored in poor conditions (too warm, too humid or exposed to light). Even taking two 25mcg tablets instead of a 50mcg of the same brand could make a difference.
You are absorbing them less effectively. They are less well absorbed if taken with anything other than plain water, or you consume anything shortly after taking them. Specifically dairy produce (which contain calcium), medicines that contain calcium, aluminium, magnesium or iron are especially significant in terms of reducing absorption.
Even taking them at a different time of day can affect absorption.
Something else is affecting the thyroid hormone within your body – such as medications like propranolol, benzodiazepine or many others. Or your diet has changed.
You for some reason now need more thyroid hormone. Maybe your thyroid now has an even lower ability to produce any thyroid hormone? Or you are now doing things that use up more thyroid hormone – like lots of exercise.
200mcg is quite a large amount so it probably represents full replacement dose. A deficit of just a few mcg per day can end up with low fT4. From memory, your body is likely to contain a total of around 2000mcg of T4 in bound form. So after, say, 200 days of taking just 10mcg less than you use, you would have very little left stored in bound form.
Even a small degree of thyroid capacity can make up that sort of gap, but if your thyroid capacity is effectively zero, you can slide down this slope without easily noticing it.
If none of the earlier suggestions apply, maybe a tiny increment of your dose to 212.5 or 225 (or maybe alternating 200 and 225) would do. For the short term you would need to take enough to get back up to sensible levels.
All the best