Can anyone explain why I keep overloading?

[Telford]

Most attachments work without its corresponding maneuver, and you're right the more maneuvers of the same element that are active the more powerful the attachments get. Such as the tension spring, with no fire maneuvers you get 3% more attack, with every subsequent fire maneuver active you get an additional 3% attack.

Other attachments require or consume maneuvers for them to work, they are:
Fire Attachments
Flame Holder
Heat Capacitor
Reactive Shield
Strobe

Ice Attachments
Ice Maker

Wind Attachments
Barrage Turbine
Replicator
Pattern Reader**

Earth Attachments
Hammermill
Schurzen
Shock Absorber
Hammermill*

Thunder Attachments
Heat Seeker
Volt Gun

Water Attachments
Condenser

Light Attachments
Auto-Repair Kit/Auto-Repair Kit II***
Eraser
Flashbulb
Vivi-Valve

Dark Attachments
Disruptor
Economizer
Mana Converter
Mana Tank/Mana Tank II***

*Shield Bash damage is increased without maneuvers but an earth maneuver is required for the slow proc.

**thought to behave like the heat seeker

***The repair kits and mana tanks will increase your puppets max HP/MP without maneuvers by using their respective maneuvers you get a regen/refresh effect.

Update: sweetidealism pointed out that volt gun does work without without a maneuver and on closer inspection found a few more attachments that require one.