While Gmail allows attachments up to 25 MBytes in size, your recipient(s) email system(s) might have a lower limit.
It is also possible that some of the attachment types are blocked:
You can't send or receive the following file types:
.ade, .adp, .bat, .chm, .cmd, .com, .cpl, .exe, .hta, .ins, .isp, .jse, .lib, .mde, .msc, .msp, .mst, .pif, .scr, .sct, .shb, .sys, .vb,.vbe, .vbs, .vxd, .wsc, .wsf, .wsh
Messages containing the types of files listed above will be bounced back and returned to the sender automatically. Gmail won't accept these file types even if they're sent in a zipped format. Here are some examples of zipped formats:
.zip, .tar, .tgz, .taz, .z, .gz, .rar
Zip file within another zip file
It isn't possible to send a zip file containing a password-protected zip file. Please de-compress all files or remove the password protection if possible.