I'm no expert, but from what I'm given to understand lots of maintenance is required, both frequent (along the lines of every few years) and of a highly technical nature (on the order of that required to build the bomb in the first place).
First, precisely because this is a super miniaturized bomb, it's not going to have a critical mass of fissile material. It therefore has to be "doped" with elements like Tritium that feed the reaction with neutrons. The suitable elements are all short lived and very difficult to obtain (more difficult even than plutonium) because they are produced in very small quantities and tightly controlled.
Secondly the radiation from the bomb deteriorates both metal and electrical components over time. The bomb must, in large measure, be "rebuilt" every x years. (I get the impression "x" is on the order of 5 to 10 years.) Again, the necessary components (e.g. super high-speed switches -- necceessary to ensure the precise timing of the conventional explosions that initiate the nuclear reaction) are not mass market items and are tightly controlled.
Basically, you have to be a major nation state with an advanced nuclear industry and an established expertise in nuclear weapons technology to maintain these things.