Sample images would be awesome - as there's a few questions here that might help you:
Why are my photos not crisp
How can I prevent blurred images when using a tripod?
How to stabilize a tripod
My own hunch is what WayneF commented: you're probably getting a bit of wobble from pressing the shutter release button. If this is actually the case, you've got options!
- Look into a remote shutter release
- Use your cameras timer mode - if there is a short and a long timer, the short should be sufficient
- Use mirror lock-up with either of the two methods above. The first shutter release will trigger the mirror to lock up. The second will trip the exposure. This allows you to add some time in between the mirror slapping and the exposure, further reducing any possible movement.
This answer, however, is highly speculative - and I'd really like to update it with something more relevant to you. So, please edit your question and add some samples to it! Thanks!