well.. Seramas are just different, to say the least. Their eggs are a lot smaller, less volume, and not as tall as standard, or bantam eggs. Seramas also are short legged so they have a hard time moving around inside of the egg. Some will internal pip, but never make it to the external pip.
Incubation is also a little different. From my experiences, lf you have a forced air type bator, 99.5 is where your temp needs to be through out. Since they have a lot less volume than others, they can be effected by thermal inversions easier. Also if you have had serama chicks before, you have seen how tiny they are. The shells on a serama egg can be just as thick as a standard, so some seramas have a harder time pipping and zipping.
Also on serama eggs they tend to, but not always, develop a little faster than any other chicken egg. You will need to lockdown on day 17 instead of 18.
There are a host of reasons seramas are harder to hatch, mostly due to size.
Here is my little bitty Tinkerbell fresh out of the egg. She's 15 weeks old now.